Ceramic filter and exhaust gas decontamination unit

ABSTRACT

It is to provide a filter for an exhaust gas having a high thermal conductivity irrespective of a relatively high porosity or showing characteristics that the whole of the filter containing a high refractive index substance or pigment is easily warmed but hardly cooled while making low the thermal conductivity of the filter as a whole. This filter is provided with a catalyst coat layer formed by carrying a catalyst active component on a surface of a porous ceramic carrier, in which a porosity of the porous ceramic carrier is 40-80% and a substance or a pigment indicating a thermal conductivity as the filter of 3-60 W/mk or having a large refractive index at a thermal conductivity of 0.3-3 W/mk.

INDICATION OF RELATED APPLICATION

This application is an application claiming a priority based on JapanesePatent Application No. 2002-96554 and No. 2002-96906 filed Mar. 29,2002.

TECHNICAL FIELD

This invention relates to a ceramic filter used in each of placesindicating various different characteristics and an apparatus for thepurification of exhaust gas using such a filter, and more particularlyto a ceramic filter formed by integrally combining a plurality of filterunits having different characteristics, or a ceramic filter formed byintegrally combining a plurality of filter units having different kindsof catalysts and carrying amounts thereof and an apparatus for thepurification for the purification of exhaust gas using such a filter.

BACKGROUND ART

Recently, the number of automobiles is exponentially increasing, and anamount of exhaust gas discharged from an internal engine of theautomobile rapidly goes on increasing. Particularly, various substancesincluding in the exhaust gas discharged from a diesel engine are a causeraising air pollution and give a serious influence upon naturalenvironment. Also, there are lately reported study results thatparticulates in the exhaust gas (diesel particulate) are a cause raisingan allergic obstacle and a reduction of spermatozoon number, so that itis an urgent issue for humans to take a countermeasure for removing theparticulates in the exhaust gas.

Under such a situation, there have hitherto been proposed variousapparatuses for the purification of the exhaust gas. As the generalapparatus for the purification of the exhaust gas, there is, forexample, a structure that a casing is arranged on the way of an exhaustpipe connected to an exhaust manifold of an engine and a filter havingfine holes is arranged therein. As a material constituting the filter,there are metals, alloys and ceramics. As a typical example of thefilter made of the ceramic is well-known a cordierite filter. Recently,porous silicon carbide sintered body having such merits that the heatresistance, mechanical strength and catching efficiency are high, andthe chemical stability is good, and the pressure loss is low and thelike is used as an example of a filter-forming material.

The general ceramic filter and the exhaust gas purification apparatususing the same are such constructed that when the exhaust gas passesthrough many cells formed along an axial direction of the filter, fineparticles in the exhaust gas are trapped in the cell walls to remove thefine particles from the exhaust gas. However, a ceramic filter made of aporous silicon carbide sintered body is generally large in the thermalexpansion and weak to thermal shock, so that there is a problem that asthe size becomes larger, crack is apt to be easily produced. In thisconnection, JP-A-8-28246 discloses a ceramic filter being an aggregateformed by combining plural small filter units as a countermeasure foravoiding the breakage due to cracks. This technique is a method whereina honeycomb shaped body of a square pole (honeycomb unit) is formed bycontinuously extruding a ceramic raw material through a die of anextrusion shaping machine and cut into an equal length to form cutpieces, and then the cut pieces are sintered to form filter units, and aplurality of such filter units are integrally bundled while adheringouter peripheral faces with each other through a ceramic adhesive toform an aggregate of these filter units as a ceramic filter.

Moreover, the ceramic filter is preferable to be wound on its outerperipheral face with a matt-shaped heat insulating material such asceramic fibers or the like, and is received in a casing disposed on theway of an exhaust pipe of an automobile or the like at such a state.

In such a filter, as the catching and burning (regeneration) of soot arerepeated, there may be caused the scattering of the catching amount ofsoot in accordance with the position of the ceramic filter.

On the other hand, JP-A-1-145377 proposes a filter of a honeycombstructure in which an average pore size is stepwise or continuouslyincreased from a partition wall of a central portion toward a partitionwall of an outer peripheral portion.

The above conventional technique (JP-A-1-145377) aims at only thecentral portion and the outer peripheral portion. In the ceramic filterhaving such a structure, when it is arranged in the casing of theexhaust pipe, there may not be carried out the uniform catching andregeneration. That is, a large amount of soot is locally andnon-uniformly caught on a part of the filter units and hence thestrength as the filter may be lowered in the regeneration or the like.Also, ash content (ash) included in a fuel additive, engine oil or thelike is apt to be easily stored on a part of the filter units and hencethe service life as the filter may be shortened.

Also, when the aforementioned conventional ceramic filter is used byrepeatedly conducting the regeneration over a constant period as awhole, as seen from a graph of a regenerating ratio (a ratio of changingweight before and after the regeneration) shown in FIG. 13, it isconfirmed that the soot is surely burnt at an initial stage, but thereactivity as a catalyst is gradually lowered as the regeneration isrepeated several times.

In the light of the above problems included in the conventionaltechniques, it is an object of the invention to provide a ceramic filterin which soot can be equally caught in axial and radial directions ofthe filter as a whole and it is effective to uniformly conduct theregeneration and the high efficiency of removing the exhaust gas (filterefficiency) can be maintained over a long time and further thedurability is excellent.

It is another object of the invention to provide an apparatus for thepurification of the exhaust gas using the above ceramic filter having ahigh filter efficiency and excellent strength and service life.

DISCLOSURE OF THE INVENTION

The inventors have made studies for achieving the above objects andconfirmed that the difference in the filter efficiency and theregeneration efficiency is caused dependent upon an arranging place ofthe ceramic filter. That is, it is confirmed that there is a largedifference in the flow amount and the flow rate between a near side andfar side of the filter to the exhaust pipe. In general, a casing 8located downstream side an exhaust manifold of a diesel engine as aninternal combustion engine is formed so as to make a diameter of itscentral section larger than those of a first exhaust pipe and a secondexhaust pipe, and the ceramic filter is received in the casing. In caseof such a ceramic filter, the gas flow amount is larger in the filterlocated at a position near to a downstream side end portion of the firstexhaust pipe than that in the filter located at a far position (flowrate is faster), and particularly when the second exhaust pipe islocated at an opposite side (lower end side), the gas flow amount andthe flow rate are more increased.

In this structure, the first exhaust pipe and the second exhaust pipeare connected so as to position at upstream side and downstream sidesandwiching the casing, so that the flow amount of the central portionof the ceramic filter becomes large (flow rate is fast) and the flowamount of the peripheral portion thereof is small (flow rate is slow).As a result, the central portion of the filter is largely affected bythe gas flown thereinto rather than the peripheral portion thereof. Inthe invention, therefore, different properties are given to each part inthe axial direction of the ceramic filter (gas flowing direction) or aradial direction perpendicular thereto, for example, different kinds offilter units dividing into the central portion having a fast flow rateand the peripheral portion having a slow flow rate are properly combinedto form a ceramic filter, whereby there can be obtained the ceramicfilter capable of conducting uniform catching and regeneration over awhole of the filter. As one concrete example, it is considered toprovide a ceramic filter formed by combining and bundling plural filterunits having different pressure loss property, strength, length and thelike and integrally adhering them.

The invention is developed under the above idea, and is a ceramic filterhaving such a basis construction that different properties are appliedto a filter made of porous ceramic sintered body having a honeycombstructure every part in a gas flowing direction (axial direction) and/ora radial direction perpendicular thereto.

(A) Particularly, a first embodiment of the invention is a ceramicfilter comprising an aggregate formed by combining a plurality of filterunits each made of a columnar, porous ceramic sintered body having ahoneycomb structure and integrally joining them (so as to bundle in aradial direction), in which said aggregate is constituted by acombination of two or more kinds of the filter units.

In the invention, it is preferable to use filter units having differentpressure loss properties as different kinds of the filter unit. As thefilter unit having the different pressure loss property, it ispreferable to use a combination of one or more kinds of either unitshaving different cell wall thicknesses, units having differentporosities, units having different pore sizes and units having differentcell structures. Concretely, a filter unit having a large pressure lossis arranged in a portion having a fast gas flow rate and a filter unithaving a small pressure loss is arranged in a portion having a slow gasflow rate, and it is preferable to integrally combine them in thefollowing forms.

-   {circle over (1)} A filter unit having a thick cell wall is arranged    in the portion having a fast gas flow rate, and a filter unit having    a thin cell wall is arranged in the portion having a slow gas flow    rate.-   {circle over (2)} A filter unit having a low porosity is arranged in    the portion having a fast gas flow rate, and a filter unit having a    high porosity is arranged in the portion having a slow gas flow    rate.-   {circle over (3)} A filter unit having a small pore size is arranged    in the portion having a fast gas flow rate, and a filter unit having    a large pore size is arranged in the portion having a slow gas flow    rate.-   {circle over (4)} A filter unit having a large cell density is    arranged in the portion having a fast gas flow rate, and a filter    unit having a small cell density is arranged in the portion having a    slow gas flow rate.

In the invention, as an example of different kinds of the filter unitscan be used filter units having different strengths. It is preferablethat a filter unit having a high strength is arranged in the portionhaving the fast gas flow rate and a filter unit having a low strength isarranged in the portion having a slow gas flow rate, but a case reversethereto may be conducted.

As an example of the different kinds of the filter units in theinvention, filter units having different lengths can be used.

According to the invention of the above construction, when a pluralityof at least two kinds of columnar filter units are combined in adirection perpendicular to an axial direction at a bundle state, thecombination can be properly changed in accordance with the situation ofexhaust gas flown into the ceramic filter as compared with a whollyuniform monolith-type ceramic filter or a ceramic filter consisting of acombination of the same kind of filter units, and hence it is possibleto conduct the uniform catching and regeneration without causingdisplacement at positions of the ceramic filter. Particularly, bycombining filter units having different properties, materials or thelike in the axial direction or radial direction of the filter, thephysical properties such as thermal conductivity and the like can bechanged in accordance with the places of the filters and hence theuniform catching and regeneration of the ceramic filter as a whole canbe ensured.

For example, by combining a filter unit having a high pressure loss witha filter unit having a low pressure loss can be easily flown the exhaustgas into the filter unit having the low pressure loss. Therefore, theaggregate as the ceramic filter can be more effectively manufactured byarranging the filter unit having a low pressure loss in places having asmall flow amount of the exhaust gas.

Under the same keystone as mentioned above, {circle over (1)} theexhaust gas in the ceramic filter can be guided toward a position of lowpressure loss by integrally combining a filter unit(s) having a highpressure loss located at a portion having a relatively fast flow rateand a filter unit(s) having a low pressure loss located at a portionhaving a relatively slow flow rate; {circle over (2)} a filter unit(s)being thin in the wall thickness and easy in the pass of the gas isarranged in place(s) having a small flow amount of the exhaust gas;{circle over (3)} a filter unit(s) having a thick wall thickness isarranged in a portion having a fast gas flow rate, while a filterunit(s) having a thin wall thickness is arranged in a portion having aslow flow rate, whereby the exhaust gas in the ceramic filter can beguided toward place having a low pressure loss; {circle over (4)} afilter unit(s) having a high porosity is arranged in place having asmall flow amount of the exhaust gas; {circle over (5)} a filter unit(s)having a low porosity is arranged in a portion having a fast flow rate,while a filter unit(s) having a high porosity is arranged in a portionhaving a slow flow rate, whereby the exhaust gas in the ceramic filtercan be guided toward place having a low pressure loss; and {circle over(6)} the filter unit having a large pore size is low in the pressureloss during and after the catching as compared with the filter unithaving a small pore size and hence it is effective to arrange the filterunit having a large pore size in place having a small flow amount of theexhaust gas. That is, when a fluid is passed through the ceramic filter,the exhaust gas in the ceramic filter can be guided toward a placehaving a low pressure loss by combinedly arranging the filter unithaving a small pore size in a portion having a relatively fast flow rateand the filter unit having a large pore size in a portion having arelatively slow flow rate.

In the above construction of the invention, as the filtering area of theceramic filter is made large by fining the cell structure of the filterunit, the pressure loss in and after the catching becomes low.Therefore, the uniform catching can be effectively conducted byarranging the filter unit having a high cell density in a portion havinga low flow amount of the exhaust gas.

When the fluid is passed through the above ceramic filter, the filterunit having a large cell density is arranged in the portion having arelatively fast flow rate and the filter unit having a small celldensity is arranged in the portion having a relatively slow flow rate,whereby the exhaust gas in the ceramic filter can be guided toward aplace in the ceramic filter having a low pressure loss of the exhaustgas.

According to the invention, there can be used filter units havingdifferent strengths. That is, a filter unit having a high strength isused in a portion of the ceramic filter subjected to thermal shock,whereby the strength as the aggregate of the ceramic filter can beimproved.

During the studies for achieving the above object, the inventors havefurther made various experiments for solving the cause of thedeterioration of the filter performance. For example, there is made atest for increasing a catalyst carrying amount of the filter. In thiscase, however, the pressure loss of the filter becomes higher and theprogress of the deterioration can not be prevented. Then, temperaturesat plural places are measured by inserting a thermocouple into thefilter to be regenerated. Contrary to expectation, it has been confirmedthat a temperature difference is caused in positions of the filter, forexample, between the central portion and outer peripheral portion. Thatis, the temperature of the filter in the central portion is higher thanthat of the filter in the outer peripheral portion.

Now, the temperature is measured by changing a connecting relationshipbetween the filter and the casing as shown in FIG. 12. As a result, thetemperature distribution appearing in the filter differs in accordancewith the difference in the connecting position between the exhaust pipeand the casing as shown in FIG. 12, and particularly the filtertemperature tends to become high in the portion near to the exhaustpipe. Therefore, when conducting the temperature control of the filter,if it is intended to control the temperature to about 600° C. wellpromoting the reaction of the catalyst, it is confirmed that a portionrendering into a high temperature exceeds 800° C. Also, as theconstruction of the catalyst itself is examined, it is confirmed that anoble metal used as the catalyst causes sintering (metal changes intolarge particles) immediately above 800° C. and the reactivity becomesbad and the use thereof is impossible.

From these facts, the inventors have knowledge that when the ceramicfilter is constituted with an aggregate of columnar filter unitsdividable in a radial direction as mentioned above, it is effective tocarry different catalysts on the respective columnar filter units orchange a carrying amount of the catalyst on the filter unit.

(B) That is, a second embodiment of the invention is a ceramic filtercomprising an aggregate formed by combining a plurality of columnarfilter units each having a honeycomb structure and made of porousceramic sintered body through a sealing material layer (preferably asealing material indicating an adhesiveness: adhesive layer) andintegrally joining them, in which the aggregate is constituted bycombining two or more kinds of the filter units and a catalyst ofdifferent carrying amount or different kind is carried on each of thefilter units.

In this embodiment, as the different kinds of the filter units carryingthe catalyst of different carrying amount or different kind can be usedone or more of the filter units having different pressure lossproperties, the filter units having different strengths and the filterunits having different lengths as mentioned above.

As the filter units having different pressure loss properties can beused one or more of units having different thicknesses of cell wall,units having different porosities, units having different pore sizes andunits having different cell structures.

In the invention, catalysts having different heat resistance orcatalytic activities can be used as the different kinds of the catalyst.

As the applying embodiment of the invention, it is preferable that whena high-temperature gas is flown in the ceramic filter, a filter unitcarried with a catalyst having a good heat resistance is arranged in aportion having a fast gas flow rate or a large gas flow amount and/or afilter unit carried with a catalyst having a poor heat resistance isarranged in a portion having a slow gas flow rate or a small gas flowamount and they are integrally combined to form a ceramic filter.

Alternatively, it is preferable that when a high-temperature gas isflown in the ceramic filter, a filter unit carried with a catalysthaving a large activity is arranged in a portion having a fast gas flowrate or a large gas flow amount and/or a filter unit carried with acatalyst having a small activity is arranged in a portion having a slowgas flow rate or a small gas flow amount.

In the invention, it is further preferable to combine a plurality offilter units having different carrying amounts of the catalyst andintegrally join them.

Furthermore, in the invention, it is preferable that when the ceramicfilter is set in a casing of an exhaust gas purifying apparatus and ahigh-temperature gas is flown therein, a filter unit having a smallcarrying amount of a catalyst is arranged in a portion having a fast gasflow rate or a large gas flow amount and/or a filter unit having a largecarrying amount of a catalyst is arranged in a portion having a slow gasflow rate or a small gas flow amount.

In the invention, the ceramic filter may be an aggregate formed byintegrally joining a plurality of filter units each made of columnar andporous ceramic sintered body having a honeycomb structure through asealing material layer, in which the aggregate is constituted with acombination of two or more different kinds of the filter units and thefilter units are an integral combination of a unit not carrying thecatalyst and a unit carrying at least one kind of catalysts.

In the invention, it is particularly preferable that when thehigh-temperature gas is flown in the ceramic filter, the filter unitcarrying the catalyst is arranged in a portion having a slow gas flowrate or a small flow amount.

The invention having the above construction has the function and effectas mentioned below. That is, in the invention, all of the filter unitfundamentally combined as the ceramic filter do not carry the samecatalyst, but the different catalyst is carried every the filter unit,so that there can be provided a ceramic filter suitable for an exhaustgas purifying apparatus of an automobile causing a deflected gasflowing.

For example, an example of arranging catalysts having different heatresistances in accordance with positions in the radial direction of thefilter or an example of carrying catalysts having different catalyticactivities means that the catalyst most suitable in the use of theceramic filter is selectively attached to the respective position.

Particularly, when the ceramic filter is set in the casing of theexhaust gas purifying apparatus and the high-temperature exhaust gas isflown thereinto, the filter carried with the catalyst having anexcellent heat resistance is used in a portion having a fast flow rateor a large gas flow amount of the exhaust gas, whereby the catalyst in aposition exposed to the high-temperature portion can be used over a longtime.

Similarly, when the high-temperature gas is flown into the ceramicfilter, the regeneration can be efficiently conducted even in a portionof a relatively low temperature by using the filter carried with thecatalyst having a large activity in the portion having a slow gas flowrate or a small gas flow amount.

Also, in the invention, the carrying amount of the catalyst may bechanged in accordance with the place in the radial direction of thefilter (the same catalyst amount may not be carried on the filter). Inthis case, it is possible to adjust the catalyst weight or density to becarried on the filter in the use of the ceramic filter. According tosuch a structure, it is not necessary to carry the same catalyst amounton the ceramic filter. In the use of the ceramic filter, therefore, itis possible to arrange the filter unit having catalyst weight anddensity adjusted to a large level in a portion having a relatively lowtemperature and arrange the filter unit having catalyst weight anddensity adjusted to a small level in a portion having reversely arelatively high temperature.

Further, according to the invention, the aggregate of the ceramic filtercan be formed by using both a filter unit carried with the catalyst anda filter unit not carried with the catalyst. Thus, the filter unithaving no catalyst can be used in a portion promoting the regenerationof soot without the catalyst at a relatively high temperature, while thefilter unit having the catalyst can be used in a portion requiring thecatalyst effect at a relatively low temperature.

In this case, it is possible to arrange the filter unit having thecatalyst in a portion having a small flow amount of the exhaust gas andbeing relatively low in the temperature and reversely arrange the filterunit having no catalyst in a portion having a large flow amount of theexhaust gas and being relatively high in the temperature in the use ofthe ceramic filter.

According to the invention, the ceramic filter having the aboveproperties can be interposed between the first and second exhaust pipesto form an exhaust gas purifying apparatus having a high strength andcapable of using over a long time.

Furthermore, according to the invention, when the fluid is passedthrough the ceramic filter, a filter unit having a high strengtharranged in a portion having a relatively fast flow rate and a filterunit having a low strength arranged in a portion having a relativelyslow flow rate are integrally combined, whereby there can be provided aceramic filter capable of coping with the rapid temperature change ofthe exhaust gas.

In addition, according to the invention, when the fluid is passedthrough the ceramic filter, a filter unit having a low strength arrangedin a portion having a relatively fast flow rate and a filter unit havinga high strength arranged in a portion having a relatively slow flow rateare integrally combined, whereby there can be provided a ceramic filtercapable of coping with the temperature change generated when abnormalburning is caused due to unburnt soot in the portion having the slowflow rate. These ceramic filters can be properly selected and used inaccordance with the specification of the engine and the regenerationsystem.

Further, according to the invention, the ceramic filter may be formed bychanging a filter length and arranging a filter unit having a shortlength in a portion having a small catching amount of soot. In theceramic filter having such a construction, the position of soot amountfrom front edge face can be made same. As a result, the aggregate of thefilters can promote the catching of soot and regeneration as whole.

Moreover, in the invention, a ceramic filter having a high filteringefficiency, a low pressure loss and excellent heat resistance andthermal conductivity can be provided by using a porous silicon carbidesintered body as a material for the ceramic filter.

(C) As a third embodiment of the invention, there is proposed an exhaustgas purifying apparatus having a high strength and capable of using overa long time by interposing and arranging various ceramic filters havingdifferent properties in an axial direction and/or a radial direction ina casing between first and second exhaust pipes.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic view illustrating an example of the exhaust gaspurifying apparatus.

FIG. 2 is a perspective view of a block formed by combining filterunits.

FIG. 3 is an enlarged section view of a main part of an exhaust gaspurifying apparatus.

FIG. 4 is an explanatory view of a pressure loss property.

FIG. 5 is a graph showing results of Reference Example indicating adifference of soot catching amount in accordance with a difference ofarranging positions of a casing and an exhaust pipe.

FIG. 6 is a graph showing results of Comparative example indicating adifference of soot storing amount by changing a shape of a filter.

FIG. 7 is a graph showing results of Comparative Example indicating adifference of soot storing amount by changing a length of a filter.

FIG. 8 is a view showing an example of carrying alumina on a ceramicfilter of an embodiment of the invention.

FIG. 9 is a graph showing a relation between specific surface areaexerting upon a heat resistance of a catalyst and a heat treating time.

FIG. 10 is a graph showing a relation between a regeneration ratio and atemperature exerting upon an activating temperature of a catalyst.

FIG. 11 is a schematically section view of an example of carrying acatalyst.

FIG. 12 is a section view illustrating a way of temperature propagationby changing arranging places of a casing and an exhaust pipe.

FIG. 13 is a graph showing a regeneration ratio after the use severaltimes.

BEST MODE FOR CARRYING OUT THE INVENTION

An exhaust gas purifying apparatus 1 for a diesel engine as anembodiment of the invention and a ceramic filter used therefor areexplained with reference to FIG. 1 to FIG. 13 below.

The exhaust gas purifying apparatus 1 shown in FIG. 1 is an apparatusfor purifying an exhaust gas discharged from a diesel engine 2 as aninternal combustion engine. The diesel engine 2 comprises pluralcylinders not shown, and each cylinder is connected to a branched part 4of an exhaust manifold 3 made of a metallic material. Each branched part4 is joined to a main body 5 of the manifold. Therefore, exhaust gasdischarged from the each cylinder is collected to the manifold mainbody.

To a downstream side of the exhaust manifold 3 are arranged a firstexhaust pipe 6 and a second exhaust pipe 7 each made of a metallicmaterial. An end portion of an upstream side of the first exhaust pipe 6is connected to the manifold main body 5, and a cylindrical casing 8made of a metallic material is arranged between the first exhaust pipe 6and the second exhaust pipe 7. That is, an end portion of an upstreamside of the casing 8 is connected to an end portion of a downstream sideof the first exhaust pipe 6, and an end portion of a downstream side ofthe casing 8 is connected to an end portion of an upstream side of thesecond exhaust pipe 7. In other words, the casing 8 is arranged at aninterposing state between the exhaust pipes 6, 7. As a result, internalregions of the first exhaust pipe 6, casing 8 and second exhaust pipe 7are communicated with each other and an exhaust gas is flown thereinto.A section at a central portion of the casing 8 is made larger thandiameters of the exhaust pipes 6, 7, so that the internal region of thecasing 8 is wider than the internal regions of the exhaust pipes 6, 7.In the casing 8 is received a ceramic filter 9.

A heat insulating material 10 is interposed between an outer peripheralface of the ceramic filter 9 and an inner peripheral face of the casing8. The heat insulating material 10 is a matt containing ceramic fibersand has a thickness of 2 mm-60 mm. It is desirable that the heatinsulating material 10 has an elastic structure and a function ofreleasing heat stress. The heat insulating material prevents the escapeof heat from the outermost peripheral portion of the filter 9, wherebyenergy loss in the regeneration can be suppressed to a minimum, and alsosince it has the elastic structure, the shifting of the position of theceramic filter 9 due to the pressure of the exhaust gas and thevibration during the running can be prevented.

In this embodiment, a main application of the ceramic filter 9 is adiesel particulate filter (DPF) for removing diesel particulates in theexhaust gas discharged from the diesel engine. A block 9 constitutingthe filter shown in FIG. 2 is formed by bundling a plurality of ceramicfilter units F1 having a honeycomb structure so as to combine facesperpendicular to an axial direction with each other and integrallyuniting them to form an aggregate (block). A filter unit F1 positionedin a central portion of the aggregate is quadratic prism-shaped, anouter size of which is 33 mm×33 mm×150 mm. In the illustratedembodiment, quadratic prism-shaped filter units F1 are bundled in 4 rowsand 4 columns, i.e. a total of 16 units to form a quadratic prism-shapedaggregate (ceramic filter) of honeycomb filter units as a whole.

These filter units F1 are preferable to be made of a porous siliconcarbide sintered body. Because, the silicon carbide sintered body isexcellent in the heat resistance and thermal conductivity as comparedwith the other ceramics. Of course, sintered bodies of silicon nitride,sialon, alumina, cordierite, mullite and the like can be used as asintered body other than silicon carbide. Further, as the unit F1 can beused a silicon-containing ceramic formed by compounding metallic siliconwith the above ceramic, or a ceramic bonded with silicon or a silicatecompound.

Also, each of these filter units F1 is provided with a honeycombstructure, so that there is an advantage that even when the catchingamount of particulates increases, the pressure loss is small. In each ofthe filter units F1 are regularly formed plural through-holes 12 havingsubstantially a quadrate at section along an axial direction of theunit. The through-holes 12 are partitioned with each other through thincell walls 13, and an oxidation catalyst consisting of an element ofplatinum group (e.g. Pt or the like) or the other metal element or anoxide thereof is carried on the cell wall 13. Of course, a catalystpurifying Ca, HC, Nox or the like may be carried, or a rare earthelement, alkaline earth metal, alkali metal, transition metal may becarried.

As shown in FIG. 13, opening portions of each through-hole 12 are sealedwith a plug 14 (porous silicon carbide sintered body in this embodiment)at either one of end faces 9 a, 9 b, and viewing the whole of the endfaces 9 a, 9 b, the opening portions and the sealed portions areconstructed so as to indicate a checkered pattern.

As a result, the filter unit F1 is at a state of constituting with manycells having a quadratic form at section. In such a cell structure, adensity of the cells is about 100-400 cells/inch square and a thicknessof the cell wall 13 is about 0.05-0.5 mm.

Moreover, the cell structure is generally represented by dividing thethickness of the cell wall 13 as a unit of mil (1 mil is 0.0254 mm) bythe cell density. That is, the above example is represented by 14/200.Therefore, about a half of many cells are opened at the upstream sideend faces 9 a, and the remaining half are opened at the downstream sideend faces 9 b.

In the filter unit F1, the average pore size is 1 μm-50 μm, preferablyabout 5 μm-20 μm. When the average pore size is less than 1 μM, theclogging of the filter unit F1 due to the deposition of the particulatesbecomes conspicuous. While, when the average pore size exceeds 50 μm,fine particulates can not be caught and the filtering ability lowers.

The porosity of the filter unit F1 is 30%-80%, preferably 35%-70%. Whenthe porosity is less than 30%, the unit becomes too dense and there is afear that the exhaust gas can not be flown into the interior of theunit. While, when the porosity exceeds 80%, the pores in the filter unitF1 becomes too large and there is a fear that the strength becomes weakand the catching efficiency of the particulates lowers.

In the ceramic filter 9 placed in the casing as shown in FIG. 3, outerperipheral faces of 16 ceramic filters 9 in total are adhered with eachother through ceramic sealing materials (preferably using an adhesionsealing material (adhesive)) 15. The sealing material layer 15 ispreferable to have a thickness of about 0.3 mm-3 mm, further preferably0.5 mm-2 mm. When the thickness is exceeds 3 mm, even if the thermalconductivity is high, the sealing material layer 15 is still a largeheat resistor and hence the thermal conduction between the filter unitsF1 is obstructed. Also, a ratio of the filter unit F1 portion occupiedin the ceramic filter 9 is relatively decreased to bring about thelowering of the filtering ability. Inversely, when the thickness of thesealing material layer 15 is less than 0.3 mm, the large heat resistoris not formed, but the adhesion property between the filter units F1 islacking and the ceramic filter 9 is easily broken.

The sealing material layer 15 is made of, for example, inorganic fibers,an inorganic binder, an organic binder and inorganic particles, and isdesirable to be made of an elastic material formed by bindingthree-dimensionally crossed inorganic fibers and inorganic particlesthrough the inorganic binder and organic binder. As the inorganic fibermay be used one or more ceramic fibers selected from silica-aluminafibers, mullite fibers, alumina fibers and silica fibers. Among them,the use of the silica-alumina ceramic fibers is desirable. Because, thesilica-alumina ceramic fiber is excellent in the elasticity and has anaction of absorbing heat stress.

In this case, the content of the silica-alumina ceramic fibers occupiedin the sealing material layer 15 is 10% by weight-70% by weight,preferably 10% by weight-40% by weight, more preferably 20% byweight-30% by weight as a solid content. When the content is less than10% by weight, the function and effect as the elastic body lower. While,when the content exceeds 70% by weight, not only the thermalconductivity but also the elasticity lower.

The silica-alumina ceramic fiber is preferable to contain shots of 1% byweight-10% by weight, preferably 1% by weight-5% by weight, morepreferably 1% by weight-3% by weight. When the shot content is less than1% by weight, the production is difficult, while when it exceeds 50% byweight, The outer peripheral face of the filter unit F1 is damaged.

The silica-alumina ceramic fiber has a fiber length of 1 mm-100 mm,preferably 1 mm-50 mm, more preferably 1 mm-20 mm. When the fiber lengthis less than 1 mm, the elastic structural body can not be formed, whilewhen the fiber length exceeds 100 mm, the fibers are pilled and thedispersibility of the inorganic particles is deteriorated, and furtherit is difficult to thin the sealing material layer 15 to not more than 3mm and hence it is not attempted to improve the thermal conductionbetween the filter units F1.

As the inorganic binder included in the sealing material layer 15 isdesirable at least one colloidal sol selected from silica sol andalumina sol. Particularly, it is desirable to select the silica sol.Because, the silica sol is available and easily changes into SiO₂through firing and is suitable as an adhesive at a high temperatureregion. Also, the silica sol is excellent in the insulating property.

Furthermore, the content of the silica sol in the sealing material layer15 is 1% by weight-30% by weight, preferably 1% by weight-15% by weight,more preferably 5% by weight-9% by weight as a solid content. When thecontent is less than 1% by weight, the adhesion strength lowers, whilewhen the content exceeds 30% by weight, the thermal conductivity largelylowers.

As the organic binder contained in the sealing material layer 15, ahydrophilic organic high polymer is preferable, and at least onepolysaccharide selected from polyvinyl alcohol, methylcellulose,ethylcellulose and carboxymethyl cellulose is more preferable. Amongthem, it is desirable to select carboxymethyl cellulose. Because,carboxymethyl cellulose gives a preferable fluidity to the sealingmaterial layer and indicates an excellent adhesiveness at a roomtemperature region. In this case, the content of the carboxymethylcellulose in the adhesive layer is 0.1% by weight-5.0% by weight,preferably 0.2% by weight-1.0% by weight, more preferably 0.4% byweight-0.6% by weight as a solid content. When the content is less than0.1% by weight, migration can not be sufficiently controlled.

Moreover, the term “migration” used herein means a phenomenon that incase of curing the sealing material layer filled between bodies to beadhered, the binder moves accompanied with the removal of the solventthrough drying. While, when the content exceeds 5.0% by weight, theorganic binder burns out at a higher temperature and the strength of thesealing material layer 15 lowers.

As the inorganic particles included in the sealing material layer 15 ispreferable an elastic material using one or more inorganic powder orwhisker selected from silicon carbide, silicon nitride and boronnitride. Such carbide and nitride are very large in the thermalconductivity and contribute to improve the thermal conduction owing tothe inclusion on a surface of the ceramic fiber or a surface of thecolloidal sol and interiors thereof. Among the inorganic powder of thecarbide and nitride, it is desirable to select silicon carbide powder.Because, silicon carbide is very high in the thermal conductivity andhas a property of being easily familiar with the ceramic fiber. Further,the filter unit F1 as a body to be adhered is the same kind, i.e. poroussilicon carbide in this embodiment.

In this case, the content of silicon carbide powder is 3% by weight-80%by weight, preferably 10% by weight-60% by weight, more preferably 20%by weight-40% by weight as a solid content. When the content is lessthan 3% by weight, the thermal conductivity of the sealing materiallayer 15 lowers and hence the sealing material layer 15 becomes still alarge heat resistor. While, when the content exceeds 80% by weight, theadhesion strength at high temperatures lowers.

In the silicon carbide powder, the particle size is 0.01 μm-100 μm,preferably 0.1 μm-15 μm, more preferably 0.1 μm-10 μm. When the particlesize exceeds 100 μm, the adhesion force and thermal conductivity lower,while when the particle size is less than 0.01 μm, the cost of thesealing material layer 15 increases.

The production method of the above ceramic filter is described below.

-   {circle over (1)} At first, there are previously provided a ceramic    starting slurry used in an extrusion shaping step, a plug paste used    in an end face sealing step, and a sealing material paste used in a    filter adhering step. The ceramic starting paste is used by    compounding given amounts of an organic binder and water with    silicon carbide powder and mixing them. The plug paste is used by    compounding an organic binder, a lubricant, a plasticizer and water    with silicon carbide powder and mixing them. The paste for the    formation of the sealing material layer is used by compounding given    amounts of inorganic fibers, an inorganic binder, an organic binder,    inorganic particles and water and mixing them.-   {circle over (2)} Then, the ceramic starting slurry is charged into    an extrusion shaping machine and continuously extruded into a mold.    Thereafter, the extruded honeycomb shaped body is cut at an equal    length to obtain cut pieces of a quadratic, columnar honeycomb    shaped body. Thereafter, a given amount of the plug paste is filled    in one-sided opening portion of a cell of the cut piece to seal both    end faces of each of the cut pieces.-   {circle over (3)} Next, the firing is carried out under given    conditions of temperature, time and the like to completely sinter    the cut pieces of the honeycomb shaped body and the plug 14.    Moreover, in order to provide an average pore size of 6 μm-15 μm and    a porosity of 35%-70%, the firing temperature is set to 2100°    C.-2300° C. and the firing time is set to 0.1 hour-5 hours, and an    atmosphere inside a furnace in the firing is an inert atmosphere and    a pressure of the atmosphere is an ordinary pressure.-   {circle over (4)} If necessary, an underground layer made of a    ceramic is formed on an outer peripheral face of the honeycomb    filter unit F1, and thereafter the paste for the formation of the    sealing material layer is applied thereonto. The thus obtained    filter unit F1 is used in total of 16 units and their outer    peripheral faces are adhered with each other to form a columnar    aggregate as an objective ceramic filter. Of course, the ceramic    filter may be subjected to a working into a target shape such as    column, ellipsoid or the like and coated with a sealing material.

Next, an action of catching particulates through the ceramic filter 9received in the casing 8 will be described. In the ceramic filter 9, theexhaust gas is fed from a side of the upstream side end face 9 a. Thatis, the exhaust gas flown from the first exhaust pipe 6 first flows intothe cell opening to the upstream side end face 9 a of the ceramicfilter. Then, the exhaust gas passes through the cell wall 13 and flowsinto the adjoining other cell (i.e. adjoining cell at a side opening tothe downstream side end face 9 b), which is discharged from the openingdownstream side end face 9 b. In such a flowing of the gas, particulatesincluded in the exhaust gas (diesel particulate) can not pass throughthe cell wall 13, so that they are trapped on the surface of the cellwall 13 as they are. As a result, the purified exhaust gas is dischargedfrom the downstream side end face 9 b of each filter unit F1. Then, thepurified exhaust gas passes through the second exhaust pipe 7 and isfinally discharged into air.

Moreover, the particulates trapped by the cell wall 13 are burnt out bythe action of the catalyst when the internal temperature of the ceramicfilter 9 reaches to a given temperature.

Then, the ceramic filter (A) according to the invention is describedwith respect to an example of the aggregate formed by combining filterunits F1 having different properties in the radial direction of thefilter, e.g. different pressure loss properties, but the invention isnot limited to this example.

In general, the pressure loss property when the exhaust gas passesthrough the cell wall 13 is considered as follows. The pressure losswhen the diesel exhaust gas passes through the filter unit F1 can beshown in FIG. 4. In this case, resistances ΔP1, ΔP2 and ΔP3 are valuesdependent upon the cell structure of the filter unit F1, respectively,and a total value of Δpi=(ΔP1+ΔP2+ΔP3) is a constant value not dependentupon the deposited amount and time of the diesel particulates, which iscalled as an initial pressure loss. On the other hand, ΔP4 is aresistance when the diesel particulates pass through the cell wall 13 ofthe filter unit F1 after the deposition, which is a value correspondingto 2-3 times or more of the initial pressure loss.

-   {circle over (1)} When the cell wall 13 is thickened, the resistance    ΔP3 passing through the cell wall 13 increases as the pressure loss.    Further, the opening becomes small and hence ΔP1 becomes large. As a    result, the pressure loss becomes considerably large as compared    with the filter having a thin cell wall 13, and the tendency thereof    becomes more conspicuous when the particulates are deposited on the    filter.-   {circle over (2)} In case of increasing the porosity, the resistance    ΔP3 passing through the cell wall 3 lowers as the pressure loss.    Therefore, the pressure loss becomes small as compared with the    filter having a low porosity, and the tendency thereof becomes more    conspicuous when the particulates are deposited on the filter.-   {circle over (3)} In case of increasing the pore size, the    resistance ΔP3 passing through the cell wall 13 lowers as the    pressure loss. Therefore, the pressure loss becomes small as    compared with the filter having a low pore size, and the tendency    thereof becomes more conspicuous when the particulates are deposited    on the filter.-   {circle over (4)} In case of making the cell density large, the    resistance ΔP1 based on the narrowing of the opening in each cell    path and the resistance ΔP2 passing through the fine pipe increases    as the pressure loss. However, the resistance ΔP4 in case of passing    the deposited soot considerably lowers, so that the pressure loss    becomes small as compared with the filter having a low cell density.

In the invention, therefore, it is considered to combine different kindsof filter units F1 (cell wall thickness, porosity, pore size, cellstructure) in accordance with loss property of each filter unit F1constituting the ceramic filter 9. For example, the filter unit F1having a large pressure loss is arranged in a portion having a fast gasflow rate and the filter unit F1 having a small pressure loss isarranged in a portion having a slow gas flow rate, and they areintegrally combined. Concrete examples thereof are mentioned as follows.

-   {circle over (1)} A filter unit having a thick cell wall is arranged    in the portion having a fast gas flow rate, and a filter unit having    a thin cell wall is arranged in the portion having a slow gas flow    rate.-   {circle over (2)} A filter having a low porosity is arranged in the    portion having a fast gas flow rate, and a filter unit having a high    porosity is arranged in the portion having a slow gas flow rate.-   {circle over (3)} A filter unit having a small pore size is arranged    in the portion having a fast gas flow rate, and a filter unit having    a large pore size is arranged in the portion having a slow gas flow    rate.-   {circle over (4)} A filter unit having a large cell density is    arranged in the portion having a fast gas flow rate, and a filter    unit having a small cell density is arranged in the portion having a    slow gas flow rate.

Also, in the invention, filter units having different strengths can beused as an example of different kinds of filter units F1. In this case,a filter unit having a high strength is arranged in the portion having afast gas flow rate and a filter unit having a low strength is arrangedin the portion having a slow gas flow rate in accordance with thespecification of the engine and the regeneration system, or thearrangement of these filter units is opposite, so that the combinationthereof can be properly selected.

Furthermore, in the invention, honeycomb filters having differentlengths can be used as an example of different kinds of the filter unitsF1.

Then, the ceramic filter (B) according to the invention is describedwith respect to an example of the aggregate formed by combining filterunits F1 having different kinds of catalyst carried on the filter unitF1 and/or different carrying amounts, but the invention is not limitedto this example. The catalyst formed on the each filter unit F1 isdescribed below.

The catalyst used in the invention (B) is preferable to be a rare earthoxide-containing alumina film 17 separately coating the surface of eachSiC particle 16 with respect to SiC particles 16 of SiC sintered bodyconstituting the cell walls 13 partitioning each cell of the filter unitF1.

In general, alumina has a high specific surface area and is suitable asa catalyst carrying film. Particularly, it is desirable to developfilter units F1 stably acting at a higher temperature and having a highheat resistance at the present time, and the alumina film 17 is requiredto have a higher heat resistance accompanied therewith.

In the invention, in order to improve the alumina film, {circle over(1)} the shape of each alumina particle is rendered into a small fiberand {circle over (2)} a rare earth oxide such as ceria (cerium oxide) orthe like is included therein. Particularly, by adopting the structure ofthe former {circle over (1)} can be reduced contact points betweenalumina particles and the sintering rate can be lowered to suppress thegrowth of particles and make the specific surface area large to therebyimprove the heat resistance. That is, in this embodiment, the aluminafilm 17 covering the surface of each SiC particle in the filter unit F1shows a hair implant structure that alumina particles stand up in formof small fibers at a microscopic section. In such a filter unit F1,therefore, the heat resistance is considerably improved because thecontact points between adjoining alumina small fibers is reduced. In thelatter {circle over (2)}, the heat resistance is improved by addingceria or the like. This is due to the fact that a new compound is formedon the surfaces of the crystal particles constituting the alumina film17 to obstruct the growth of alumina particles.

In this embodiment, SiC or SiO₂ existing on an extreme surface layerthereof feeds Si in the heat treatment to shut off a mass transfercourse, whereby the heat resistance is improved. According to theinventors' studies, it is confirmed that when SiC is intentionallytreated at a high temperature to form an oxide, the heat resistance isfurther improved.

The regeneration property of the alumina film 17 is described below. Thealumina film 17 is obtained by adding a rare earth oxide such as ceria(CeO₂) or lanthana (La₂O₃) in an amount of about 10-80 mass %,preferably 20-40 mass % based on Al₂O₃ and uniformly dispersing theoxide into the surface or interior of the alumina film 17. As ceria orthe like is added to the alumina film 17 (it is desirable to add acatalyst such as Pt or the like together), the feeding of oxygen intothe exhaust gas is activated by an action of adjusting oxygenconcentration inherent to ceria, whereby the efficiency of burning andremoving “soot (diesel particulates) adhered to the filter unit F1 isimproved and hence the regeneration ratio of the filter unit F1 isconsiderably improved. Also, the durability of the filter unit F1 can beimproved.

That is, the rare earth oxide such as ceria or the like improves theheat resistance of alumina but also plays a role of adjusting the oxygenconcentration on the surface of the filter unit F1. In general,hydrocarbon or carbon monooxide existing in the exhaust gas is removedby oxidation reaction and NOx is removed by reducing reaction. Ceriaadded to the catalyst is relatively low in the redox potential of Ce³⁺and Ce⁴⁺ and reversibly promotes a reaction of 2CeO₂⇄Ce₂O₃+1/2O₂. Thatis, when the exhaust gas renders into a rich region, the above reactionproceeds to the right to feed oxygen into an atmosphere, while when itrenders into a lean region, the reaction proceeds to the left to occludeextra oxygen in the atmosphere. Thus, the ceria adjusts the oxygenconcentration in the atmosphere and takes an action of widening a widthof an air-fuel ratio so as to efficiently remove hydrocarbon, carbonmonooxide or NOx.

On the contrary, the catalyst containing no CeO₂ oxidizes soot byactivating oxygen in the exhaust gas. This reaction is poor in theefficiency because oxygen in the fluid should be activated.

On the other hand, the catalyst containing CeO₂ feeds oxygen by thefollowing reaction formula:CeO₂⇄CeO_(2-x)+x/20₂That is, oxygen discharged into the atmosphere and oxygen in the exhaustgas are activated by the catalyst (noble metal) and reacted with soot(carbon) to form CO₂ (CeO_(2-x) is oxidized to original CeO₂). Also,CeO₂ and soot directly contact with each other, so that even if theoxygen amount discharged is small, the soot can be efficiently oxidized.

Further, CeO₂ increases OSC (oxygen storing capacity) owing to thecarrying of the catalyst (noble metal). Because, the catalyst (noblemetal) activates oxygen in the exhaust gas and activates oxygen on thesurface of CeO₂ near to noble metal, and the OSC is increased.

Moreover, as the rare earth oxide, it is preferable to use a compositeoxide of rare earth element and zirconium in addition to the abovesingle oxide (CeO₂). In this case, it is considered that when zirconiumoxide is included in the rare earth oxide, the grain growth of the rareearth oxide is suppressed to improve the controlling property of oxygenconcentration.

In the rare earth oxide forming a composite oxide with zirconium, theparticle size is preferably about 1-30 nm, more preferably 2-20 nm. Whenthe particle size is less than 1 nm, it is difficult to produce thecomposite oxide. While, when the particle size exceeds 30 nm, theparticles easily cause the sintering and hence the surface area of theparticles becomes small and the contact area with the exhaust gasbecomes small and there is a problem that the activity is weak. Further,there is feared that the pressure loss in the pass of the exhaust gasbecomes large.

Onto the filter unit F1 showing a state that the surface of the filterunit F1 is apparently coated with the alumina film (carrying film) 17 byindependently covering each surface of the SiC particles 16 with thealumina film 17 is carried a noble metal or an element selected fromelements of Group VIa and VIII in the Periodic Table as a catalyst. Assuch an element are concretely mentioned platinum (Pt), palladium (Pd),rhodium (Rh), nickel (Ni), cobalt (Co), molybdenum (Mo), tungsten (W),cerium (Ce), copper (Cu), vanadium (V), iron (Fe), gold (Au), silver(Ag) and the like.

As the catalyst, therefore, at least one element selected from Pt, Au,Ag and Cu as a noble metal, Mo, W as an element of Group VIa, Fe, Co,Pd, Rh, Ni as an element of Group VIII and V, Ce as an element other theabove in the Periodic Table or a compound thereof may be carried on thealumina film 17.

For example, a binary alloy or a ternary alloy based on a combination ofthe above elements is used as the compound. It is advantageous to usesuch an alloy together with the rare earth oxide such as ceria, lanthanaor the like acting as a co-catalyst as mentioned above. The thus formedfilter unit F1 is less in the deterioration due to poisoning (leadpoisoning, phosphorus poisoning, sulfur poisoning) and small in thethermal deterioration and excellent in the durability. Moreover,compounds based on a combination with the other element (oxide, nitrideor carbide) may be used in addition to the alloy of the abovecombination.

Incidentally, the binary alloy includes Pt/Pd, Pt/Rh, Pt/Ni, Pt/Co,Pt/Mo, Pt/W, Pt/Ce, Pt/Cu, Pt/V, Pt/Fe, Pt/Au, Pt/Ag, Pd/Rh, Pd/Ni,Pd/Co, Pd/Mo/ Pd/W, Pd/Ce, Pd/Cu, Pd/V, Pd/Fe, Pd/Au, Pd/Ag, Rh/Ni,Rh/Co, Rh/Mo, Rh/W, Rh/Ce, Rh/Cu, Rh/V, Rh/Fe, Rh/Au, Rh/Ag, Ni/Co,Ni/Mo, Ni/W, Ni/Ce, Ni/Cu, Ni/V, Ni/Fe, Ni/Au, Ni/Ag, Co/Mo, Co/W,Co/Ce, Co/Cu, Co/V, Co/Fe, Co/Au, Co/Ag, Mo/W, Mo/Ce, Mo/Cu, Mo/V,Mo/Fe, Mo/Au, Mo/Ag, W/Ce, W/Cu, W/V, W/Fe, W/Au, W/Ag, Ce/Cu, Ce/V,Ce/Fe, Ce/Au, Ce/Ag, Cu/V, Cu/Fe, Cu/Au, Cu/Ag, V/Fe, V/Au, V/Ag, Fe/Au,Fe/Ag, Au/Ag.

As an example of the ternary alloy, there are Pt/Pd/Rh, Pt/Pd/Ni,Pt/Pd/Co, Pt/Pd/Mo, Pt/Pd/W, Pt/Pd/Ce, Pt/Pd/Cu, Pt/Pd/V, Pt/Pd/Fe,Pt/Pd/Au, Pt/Pd/Ag, Pt/Rh/Ni, Pt/Rh/Co, Pt/Rh/Mo, Pt/Rh/W, Pt/Rh/Ce,Pt/Rh/Cu, Pt/Rh/V, Pt/Rh/Fe, Pt/Rh/Au, Pt/Rh/Ag, Pt/Ni/Co, Pt/Ni/Mo,Pt/Ni/W, Pt/Ni/Ce, Pt/Ni/Cu, Pt/Ni/V, Pt/Ni/Fe, Pt/Ni/Au, Pt/Ni/Ag,Pt/Co/Mo, Pt/Co, W, Pt/Co/Ce, Pt/Co/Cu, Pt/Co/V, Pt/Co/Fe, Pt/Co/Au,Pt/Co/Ag, Pt/Mo/W, Pt/Mo/Ce/ Pt/Mo/Ce, Pt/Mo/Cu, Pt/Mo/V, Pt/Mo/Fe,Pt/Mo/Au, Pt/Mo/Ag, Pt/W/Ce, Pt/W/Cu, Pt/W/V, Pt/W/Fe, Pt/W/Au, Pt/W/Ag,Pt/Ce/Cu, Pt/Ce/V, Pt/Ce/Fe, Pt/Ce/Au, Pt/Ce/Ag, Pt/Cu/V, Pt/Cu/Fe,Pt/Cu/Au, Pt/Cu/Ag, Pt/V/Fe, Pt/V/Au, Pt/V/Ag, Pt/Fe/Au, Pt/Fe/Ag,Pt/Au/Ag.

In order to carry these catalysts on the alumina film 17, there areconsidered various methods. As a method suitable for this embodiment, animpregnation method, an evaporation drying method, an equilibriumadsorption method, incipient wetness method or a spraying method isadvantageously applicable. Among them, the incipient wetness method isadvantageous. This method is a method wherein an aqueous solutioncontaining a given amount of the catalyst is added dropwise to thefilter unit F1 and the impregnation of the catalyst into pores of thefilter unit F1 is stopped at a time of uniformly and slightly wettingthe surface of the carrier (incipient state) and thereafter the dryingand firing are conducted. That is, the dropwise addition of thecatalyst-containing solution on the surface of the filter unit F1 iscarried out by using a bullet or an injection syringe. The carryingamount of catalyst can be controlled by adjusting the concentration ofthe solution.

Then, the method of carrying the catalyst on the filter unit F1 isexplained, but it is not limited to the following method.

A feature of the method of carrying the catalyst on the filter unit F1in this embodiment lies in that the alumina film 17 containing the rareearth oxide is formed on the irregular surface of the filter unit F1 bya sol-gel process. Particularly, the alumina film 17 containing the rareearth oxide is coated onto each surface of SiC particles forming thecell wall 13 through the impregnation of the solution. Then, the aluminafilm 17 is changed into an alumina film (carrier film) 17 showing a hairimplant structure that ceria-dispersed alumina stands up in form ofsmall fibers at a microscopic section through a step of treated with hotwater after the calcining, and then a given amount of the catalyst isadsorbed and fixed onto the surface of the alumina film 17.

Each step of (1) formation of alumina film 17 and (2) carrying ofcatalyst will be described below.

(1) Coating of Alumina Film 17 onto Filter Unit F1

a. Preliminary Treating Step

In this step, oxidation is carried out by heating at 800-1600° C. for5-100 hours in order to feed a required amount of Si promoting achemical bonding with alumina on each surface of SiC particles 16. Ifsufficient oxide film is existent on the surface of SiC particles 16,this step may be naturally omitted. For example, SiC sintered bodyitself contains about 0.8 mass % of SiO₂. When SiO₂ is increased forfurther improving the heat resistance, it is desirable to heat at800-1600° C. in an oxidizing atmosphere for 5-100 hours. When thetemperature is lower than 800° C., the oxidation reaction hardly occurs,while when it exceeds 1600° C., the oxidation reaction extremelyproceeds and the lowering of the strength in the filter is caused. Arecommendable condition is 1000-1500° C. and 5-20 hours. In thiscondition, SiO₂ enough to feed Si can be formed on the surface, and alsothe porosity and pore size in the filter unit F1 are not changed and thepressure loss property is not damaged.

b. Solution Impregnation Step

In this step, a treatment coating the rare earth oxide containingalumina film 17 is carried out by impregnating each surface of the SiCparticles 16 constituting the cell wall 13 with a solution of a metalcompound containing aluminum and rare earth element, e.g. a mixedaqueous solution of aluminum nitrate and cerium nitrate or the likethrough a sol-gel process.

As to aluminum-containing compound solution in the above mixed aqueoussolution, as a starting metal compound are used a metal inorganiccompound and a metal organic compound. As the metal inorganic compoundare used Al(NO₃)₃, AlCl₃, AlOCl, AlPO₄, Al₂(SO₄)₃, Al₂O₃, Al(OH)₃, Aland the like. Among them, Al(NO₃)₃ and AlCl₃ are preferable because theyare easily soluble in alcohol, water or the like and are easy in thehandling. As the metal organic compound are mentioned a metal alkoxide,a metal acetylacetonate and a metal carboxylate. As a concrete example,there are Al(OCH₃)₃, Al(OC₂H₃)₃, Al(iso-OC₃H₇)₃ and the like.

On the other hand, as the cerium-containing solution in the above mixedaqueous solution are used Ce(NO₃)₃, CeCl₃, Ce₂(SO₄)₃, CeO₂, Ce(OH)₃,Ce₂(CO₃)₃ and the like.

As a solvent in the above mixed solution is used at least one selectedfrom water, alcohol, diol, polyvalent alcohol, ethylene glycol, ethyleneoxide, triethanolamine, xylene and the like.

In the preparation of the solution, hydrochloric acid, sulfuric acid,nitric acid, acetic acid or fluoric acid may be added as a catalyst.Further, in order to improve the heat resistance of the alumina film 17,at least one element selected from Ce, Li, K, Ca, Sr, Ba, La, Pr, Nd, Siand Zr or a compound thereof other than oxide (nitrate, chloride,sulfate, hydroxide or carbonate) may be added to the starting materialin addition to the rare earth oxide.

As an example of a preferable metal compound in this embodiment, mentionmay be made of Al(NO₃)₃ and Ce(NO₃)₃. They are dissolved in the solventat a relatively low temperature and is easy in the preparation of thestarting solution. Also, 1,3-butane diol is recommended as a preferablesolvent. A first reason of the recommendation lies in that the viscosityis adequate and a gel film having a proper thickness may be formed onthe SiC particles 16 at a gel state. A second reason is due to the fcatthat this solvent forms a metal alkoxide in the solution and a metaloxide polymer having oxygen-metal-oxygen bond or a precursor of metaloxide gel may be easily formed.

The amount of Al(NO₃)₃ is desirable to be 10-50 mass %. When it is lessthan 10 mass %, the alumina amount having a surface area for maintainingthe activity of the catalyst for a long time can not be carried, whilewhen it exceeds 50 mass %, an amount of heat generation becomes large inthe dissolution and the gelation is easily caused.

Also, the amount of Ce(NO₃)₃ is preferable to be 1-30 mass %. When it isless than 1 mass %, the oxidation of the soot can not be promoted, whilewhen it exceeds 30 mass %, the grain growth of CeO₂ occurs after thefiring.

The mixing ratio of Al(NO₃)₃ to Ce(NO₃)₃ is preferable to be 10:2. AsAl(NO₃)₃ becomes richer, the dispersibility of CeO₂ particles after thefiring can be improved.

In the preparation of the impregnating solution of the above metalcompound, the temperature is desirable to be 50-130° C. When thetemperature is lower than 50° C., the solubility of the solute is low,while when it exceeds 130° C., the reaction is violently promoted tocause gelation and the use of the applying solution is impossible.

The stirring time is desirable to be 1-9 hours. When it is within theabove range, the viscosity of the solution is stable.

As to the above cerium-containing metal compound (Al(NO₃)₃ andCe(NO₃)₃), ZrO(NO₃)₂ or ZrO₂ is used as a zirconium source for producinga composite oxide with zirconium or a solid solution in addition to theabove example. In this case, it is preferable that the above substanceis dissolved in water or ethylene glycol to form a mixed solution anddried and fired to form the above composite oxide.

In the above embodiment, it is important that the solution of the abovemetal compound is sufficiently penetrated into all pores between SiCparticles 16 in the cell wall 13. For this end, it is preferable toadopt a method wherein the filter unit F1 is placed in a vessel and thesolution of the metal compound is filled therein to conduct deaeration,a method wherein the solution is flown into one side of the filter unitF1 and deaeration is carried out at the other side, and the like. As thedeaeration apparatus may be used a vacuum pump or the like in additionto an aspirator. By using such an apparatus can be removed air from thepores in the cell wall 13 and hence the solution of the metal compoundcan be well fed onto the each surface of SiC particles 16.

c. Drying Step

This step is a treatment that volatile components such as NO₂ and thelike are removed by evaporation and the solution is gelated and fixed tothe surfaces of SiC particles 16 and at the same time extra solution isremoved, which is carried out by heating at 120-170° C. for about 2hours. When the heating temperature is lower than 120° C., the volatilecomponents hardly evaporate, while when it exceeds 170° C., the gelatedfilm thickness become non-uniform.

d. Preliminary Firing Step

This step is a calcining treatment for removing residual components toform an amorphous alumina film 17. Concretely, it is desirable to heatat a temperature of 300-500° C. When the calcining temperature is lowerthan 300° C., the residual organic mass is hardly removed, while when itexceeds 500° C., Al₂O₃ is crystallized and hence boehmite of smallfibrous protrusion can not be formed.

e. Hot Water Treating Step

This step is a treatment that the calcined filter unit F1 is immersed inhot water for forming the alumina film 17 having a specified structureinherent to the above embodiment. In this hot water treatment, theparticles on the surface of the amorphous alumina film 17 is immediatelydischarged into the solution at a sol state by a deflocculation actionand also boehmite particles produced by hydration are aggregated in formof small fibrous protrusions and form a stable state againstdeflocculation.

That is, the rare earth oxide-containing alumina adhered to each surfaceof the SiC particles 16 is stood up in form of small fibers(protrusions) by the hot water treatment, which forms a rough surfaceindicating the hair implant structure. Therefore, there is formed thealumina film 17 having a high specific surface area. In general, thesintering of alumina mainly proceeds through surface diffusion, and whenit is changed into α-alumina phase, the specific surface area rapidlyreduces. However, since silica is enclosed in the alumina particles, itis considered that hole sites of alumina are clogged with silica in thecourse of the heat treatment or silica moves toward surfaces ofneedle-like particles to control the surface diffusion or the sinteringof the particles. Therefore, it is considered that the viscous flowingmechanism through the sintering from the contact point between theneedle-like particles is predominant in the initial sintering stage ofthe filter unit F1, but silica shuts off the mass transfer path betweenthe needle-like particles at the last stage and hence the transfer intoα-alumina is obstructed and the sintering is not proceeded to maintainthe high specific surface area.

In the hot water treatment, the temperature is desirable to be 50-100°C. When it is lower than 50° C., the hydration of the amorphous aluminafilm 17 does not proceed and the boehmite of small fibrous protrusion isnot formed. While, when it exceeds 100° C., water is evaporated and thisstep is hardly maintained over a long time. The treating time isdesirable to be not less than 1 hour. When it is less than 1 hour, thehydration of the amorphous alumina is insufficient.

f. Firing Step

In this step is conducted a treatment for dehydrating boehmite producedthrough hydration to form alumina crystal. The firing temperature ispreferable to be 500-1000° C. When the temperature is lower than 500°C., the crystallization does not proceed, while when it exceeds 1000°C., the sintering proceeds much and there is a tendency of lowering thesurface area.

(2) Carrying of Ccatalyst

a. Solution Preparing Step

Onto the surface of the filter unit F1 is coated the rare earthoxide-containing alumina film (carrying film) 17 having the hair implantstructure as shown in FIG. 4(b) and a catalyst such as Pt or the like iscarried on the irregular surface of the alumina film 17. In this case,the carrying amount of the catalyst is determined by adding dropwise andimpregnating the aqueous solution containing Pt or the like by a waterabsorbing amount of the filter unit F1 so as to slightly wet the surfacethereof.

For example, the water absorbing amount held by the filter unit F1 meansthat when the measured value of the water absorbing amount of a drycarrier is 22.46 mass %, if the carrier has a mass of 110 g and a volumeof 0.1631, the carrier absorbs 24.79/1 of water. As a starting materialof Pt is used, for example, a solution of dinitrodiamine platinumnitrate ([Pt(NH₃)₂(NO₂)₂]HNO₃, Pt concentration: 4.53 mass %). In orderto carry a given amount (1.7 g/l) of Pt, it is sufficient to carry Pt of1.7 (g/l)*0.163 (1)=0.272 g on the carrier, so that the solution ofdinitrodiamine platinum nitrate (Pt concentration: 4.53%) is dilutedwith a distilled water. That is, a weight ratio X (%) of solution ofdinitrodiamine platinum nitrate (Pt concentration: 4.53 mass%)/distilled water is 24.8 mass % as calculated by X=0.272 (Pt amount,g)/24.7 (water content, g)/4.53 (Pt concentration, mass %).

b. Solution Impregnating Step

The aqueous solution of dinitrodiamine platinum nitrate adjusted to agiven amount as mentioned above is added dropwise to both end faces ofthe filter unit F1 at constant intervals through a pipette. For example,40-80 droplets are added every one-side face at constant intervals,whereby Pt is uniformly dispersed and fixed onto the surface of thealumina thin film 3 covering the filter unit F1.

c. Drying, Firing Step

After the addition of the aqueous solution, the ceramic carrier is driedat 110° C. for about 2 hours to remove water content and transferredinto a desiccator and left to stand therein for 1 hour and thereafterthe adhered amount is measured by an electron weighing machine or thelike. Then, the firing is carried out in N₂ atmosphere at about 500° C.for about 1 hour to metallize Pt. The filter unit F1 is used as a dieselparticulate filter (hereinafter abbreviated as DPF simply). This unititself has only a function of catching particulates (floating particularsubstance: PM) with the cell wall 13, but when the catalyst is carriedthereon, hydrocarbon and carbon monooxide in the exhaust gas can beoxdized.

Also, it is possible to conduct reduction of NOx even in an oxidizingatmosphere as in the diesel exhaust gas when being carried with NOxselective reduction type catalyst component or occlusion type catalystcomponent capable of reducing NOx. Moreover, the particulates caught inDPF bring about the increase of pressure loss of DPF with thedeposition, so that it is usually necessary to remove them by burningtreatment or the like to regenerate DPF. The temperature starting theburning of soot (carbon), which is a main component of the particulatesincluded in the diesel exhaust gas, is about 550-630° C. In thisconnection, when the catalyst is carried on DPF, the burning reactionpath of the soot is changed and energy barrier can be made low. Thus,the burning temperature is largely lowered to 300-400° C. and henceenergy required for regeneration can be reduced to constitute DPF systemhaving a high regeneration efficiency together with the action of ceriaas mentioned above.

As mentioned above, it can be said that the filter unit F1 according tothe embodiment of the invention is preferable to apply to a system forparticularly treating a diesel exhaust gas, whereby the followingfunctions can be expected:

a. Function as an Oxidation Catalyst for Diesel Exhaust Gas

-   -   (1) Exhaust gas purifying function . . . oxidation of THC (total        hydrocarbon), CO    -   (2) Function not obstructing operation of engine . . . pressure        loss        b. Function as a Diesel Particulate Filter Provided with a        Catalyst    -   (1) Exhaust gas purifying function . . . burning temperature of        soot, oxidation of THC, CO    -   (2) Function not obstructing operation of engine . . .        pressure Loss

EXAMPLES REFERENCE EXAMPLE

(1) 51.5% by weight of α-type silicon carbide powder and 22% by weightof β-type silicon carbide powder are mixed at a wet state and added andmixed with 6.5% by weight of an organic binder (methylcellulose) and 20%by weight of water. Then, the mixture is added and mixed with smallamounts of a plasticizer and a lubricant and extruded to obtain ahoneycomb green shaped body.

(2) Then, the green shaped body is dried by using a microwave dryingmachine. Thereafter, a part of through-holes 12 in the shaped body aresealed with a plug paste made of a porous silicon carbide sintered body.Then, the plug paste is dried by again using the drying machine.

Subsequent to the end face sealing step, the dried shaped body isdegreased at 400° C. and thereafter fired at 2200° C. in an argonatmosphere under an atmospheric pressure for about 3 hours. As a result,there is obtained a filter unit F1 made of a porous silicon carbidesintered body.

(3) A paste used for the formation of a sealing material layer 15 isprepared by mixing 23.3% by weight of ceramic fibers (alumina silicateceramic fibers, shot content: 3%, fiber length: 0.1 mm-100 mm), 30.2% byweight of silicon carbide powder having an average particle size of 0.3μm, 7% by weight of silica sol as an inorganic binder (amount of the solconverted into SiO₂ is 30%), 0.5% by weight of carboxymethyl celluloseas an organic binder and 39% by weight of water and adjusting aviscosity of the mixture.

(4) The, the paste for the formation of the adhesive layer is uniformlyapplied onto each outer peripheral face of the filter units F1, whilethe outer peripheral faces of the filter units F1 are closed to eachother and dried and cured under conditions of 50° C.-100° C.×1 hour,whereby the filter units F1 are adhered to each other through thesealing material layer 15. In this case, the thickness of the sealingmaterial layer 15 is set to 1.0 mm.

(5) The filter units F1 are combined in 16 units in total of 4 units×4units to finish a ceramic filter 9 of a square at section.

The thus obtained ceramic filter 9 is wound with a heat insulatingmaterial 10, and then the ceramic filter 9 is placed in a casing 8 atthis state, and a first exhaust pipe 6, the casing 8 and a secondexhaust pipe 7 are arranged in this order. Using casings 8 a-8 d havingfour kinds of pipe arrangements as shown in FIG. 5 is fed an exhaust gasfrom a diesel engine for a constant time. After the catching for 10hours, the ceramic filter 9 is taken out and cut to observe visually.

As a result, soot 16 is stored in the each kind of the casings 8 a, 8 b,8 c and 8 d as shown in FIG. 5. In case of the casing 8 a, the firstexhaust pipe 6 and the second exhaust pipe 7 are arranged in a centralportion of the casing 8 at section, so that a greater part of the soot16 is stored in the central portion of the ceramic filter 9. In case ofthe casing 8 b, the first exhaust pipe 6 and the second exhaust pipe 7are arranged on either one side of the peripheral edge portion of thecasing 8, so that a greater part of the soot 16 is stored on anelongated line of the exhaust pipe toward the peripheral edge portion ofthe ceramic filter 9 at section. In case of the casing 8 c, the firstexhaust pipe 6 is arranged in the central portion of the casing 8 atsection, while the second exhaust pipe 7 is arranged in the peripheraledge portion of the casing 8 at section, so that a greater part of thesoot is stored in the central portion of the ceramic filter 9 and theperipheral edge portion arranging the second exhaust pipe. In case ofthe casing 8 d, the first exhaust pipe 6 is positioned in an upperperipheral portion of the casing 8, while the second exhaust pipe 7 ispositioned in a lower peripheral portion of the casing 8 and is notopposed to the first exhaust pipe 6, so that a greater part of the soot16 is stored in portions of the ceramic filter 9 corresponding to thearranged positions of the first exhaust pipe 6 and the second exhaustpipe 7.

In the ceramic filter 9 is observed a tendency that the soot is largelystored in the filter units F1 near to the downstream side end of thefirst exhaust pipe 6 and the upstream side end of the second exhaustpipe 7.

It can be considered that the storing amount of the soot 16 isproportional to a flowing amount of a gas. For example, the filter unitF1 positioned near to the downstream side end of the first exhaust pipeis faster in the incoming gas rate as compared with the filter unit F1located far away therefrom (the amount is larger), and also if theupstream side end of the second exhaust pipe is located opposite to thesame filter, the gas rate in the filter at this position more increases(the amount becomes larger).

In the example mentioned later, therefore, the first exhaust pipe andthe second exhaust pipe are connected to the central portion of thecasing as shown in FIG. 5(a). In the ceramic filter 9, therefore, therecan be another region producing the difference of the flow rate betweenthe central portion of 4 units and the peripheral portion of 12 units.Next, comparative tests are carried out by applying various kinds offilters to the two regions.

Example 1

In Example 1-1, the ceramic filter 9 is basically prepared in the samemanner as the reference example. In Example 1-1, however, a ceramicfilter 9 of 16 filters in total is prepared by using two kinds of SiCfilter and cordierite filter as shown in Table 1 and arranging 12commercially available cordierite filters in the peripheral portion and4 SiC filters in the central portion. In this case, the thermalconductivity of cordierite is 2 W/mk, and the thermal conductivity ofSiC is 70 W/mk.

Then, the thus obtained ceramic filter 9 is wound with a heat insulatingmaterial 10 and received in a casing 8 at this state, and then theexhaust gas of the engine is fed for a constant time as shown in thereference example. After the catching for 10 hours, the ceramic filter 9is taken out and cut to conduct visual observation. As a result, thestoring amount at the peripheral portion becomes smaller than that atthe central portion as shown in FIG. 6(a).

Next, a new ceramic filter 9 is provided and the exhaust gas is fedthereto. After the catching and regeneration are repeated 100 times, theceramic filter 9 is taken out to conduct the visual observation. As aresult, no crack is observed in the filter units F1. Also, the averageregeneration ratio of the ceramic filter 9 is as high as 95%, and nosoot 16 is existent as visually observed after the cutting.

Comparative Examples 1-1,1-2

Even in Comparative Examples 1-1,1-2, the ceramic filter 9 is basicallyprepared in the same manner as in the reference example. However, inComparative Example 1-1, a ceramic filter 9 is prepared by using 16 SiCfilter units as shown in Table 1. In Comparative Example 1-2, a ceramicfilter 9 of 16 filters in total is prepared by using two kinds of SiCfilter and cordierite filter and arranging 4 cordierite filters in thecentral portion and 12 SiC filters in the outer peripheral portion.

Then, the thus obtained ceramic filter 9 is wound with a heat insulatingmaterial 10 and received in a casing 8 at this state, and the exhaustgas of the engine is fed for a constant time as shown in the referenceexample. After the catching for 10 hours, the ceramic filter 9 is takenout and cut to conduct visual observation. As a result, in ComparativeExamples 1-1 and 1-2, the storing amount in the peripheral portion ismade small likewise Example 1-1 as shown in FIG. 6(a).

Then, a new ceramic filter 9 is used and the exhaust gas is fed, andafter the catching and regeneration are repeated 100 times, the filteris taken out to conduct visual observation. As a result, no crack of thefilter unit F1 is observed in Comparative Example 1-1. However, theaverage regeneration ratio of the ceramic filter is as low as 85%, andas a result of visual observation on the cut face, a small amount ofsoot is retained in the filter units F1 located at the outer peripheralportion after the burning. In Comparative Example 1-2, cracks areobserved in the filter units F1 located at the central portion. Also,the average regeneration ratio is as low as 80%, and as a result ofvisual observation on the cut face, a greater amount of soot is retainedin the filter units F1 located at the outer peripheral portion after theburning as compared with Comparative Example 1-1. TABLE 1 ComparativeComparative Example 1-1 Example 1-1 Example 1-2 Central PeripheralCentral Peripheral Central Peripheral portion portion portion portionportion portion Filter unit SiC Cordierite SiC SiC Cordierite SiC CellStructure 14/200 14/200 14/200 14/200 14/200 14/200 (mill/cell number)Porosity of 42% 42% 42% 42% 42% 42% filter Pore size of 10 μm 10 μm 10μm 10 μm 10 μm 10 μm filter Pressure loss 10 8 10 10 8 10 (PM0 g/L, 13m/s) Average 350 kgf/cm² 25 kgf/cm² 350 kgf/cm² 350 kgf/cm² 25 kgf/cm²350 kgf/cm² bending load Regeneration 95% 85% 80% ratio

Example 2

In this example, experiments are carried out as a typical example ofceramic filters 9 having different pressure losses. The ceramic filter 9is basically prepared in the same manner as in the reference example. Inthis case, the filters are prepared by changing molds to adjust athickness of cell wall 13 in the SiC filter unit or cell number as shownin Table 2 and by changing a compounding ratio of starting materials toadjust a porosity and a pore size as shown in Table 3.

With respect to the filter units F1, a standard pressure loss ismeasured (pressure difference is measured when air of PMO g/L is flownat a flow rate of 13 m/s), and the results are also shown in Tables 2and 3.

Then, the ceramic filter 9 is prepared by setting 12 filters to theperipheral portion and setting 4 filters to the central portion as shownin Table 4.

Next, the thus obtained ceramic filter 9 is wound with a heat insulatingmaterial 10 and received in a casing 8, and an exhaust gas of an engineis fed for a given time as shown in the reference example. After thecatching for 10 hours, the filter is taken out and cut to conduct visualobservation. The results are also shown in Table 4.

Thereafter, a new ceramic filter 9 is again provided and the exhaust gasis actually fed. After the catching and regeneration of 100 times, theceramic filter 9 is taken out to conduct visual observation. The resultsand regeneration ratio are shown in Table 4. TABLE 2 A Substrate BSubstrate C Substrate D Substrate E Substrate F Substrate G Substrate H{circle over (1)} SiC SiC SiC SiC SiC SiC SiC SiC {circle over (2)}12/200 14/200 16/200 16/200 12/70 12/200 12/300 12/400 {circle over (3)}42% 42% 42% 42% 42% 42% 42% 42% {circle over (4)} 10 10 10 10 10 10 1010 {circle over (5)} 9 10 13 14 8.9 8.9 9.1 9.1 {circle over (6)} 11 1215.6 16.8 15 13 10.3 9.5{circle over (1)} Filter unit{circle over (2)} Cell structure (mill/cell){circle over (3)} Porosity of filter (%){circle over (4)} Average pore size of filter (μm){circle over (5)} Pressure loss P1 (PM 0 g/L, 13 m/s){circle over (6)} Pressure loss P2 (PM 5 g/L, 13 m/s)

TABLE 3 Substrate A Substrate I Substrate J Substrate K Substrate LFilter unit SiC SiC SiC SiC SiC Cell structure 12/200 12/200 12/20012/200 12/200 (mill/cell number) Average particle size of SiC(70  10 μm 10 μm  10 μm  30 μm  50 μm parts by weight) Average particle size of0.5 μm 0.5 μm 0.5 μm 0.5 μm 0.5 μm SiC(30 parts by weight) Shapingassistance 10 parts by 6 parts by 15 parts by 10 parts by 10 parts by(methyl cellulose) weight weight weight weight weight Amount of organicpore forming  0 parts by 0 parts by  5 parts by  0 parts by  0 parts bymaterial (acryl) weight weight weight weight weight Average pore size oforganic pore  10 μm  30 μm  50 μm forming material (acryl) Porosity offilter unit(%) 42% 30% 50% 42% 42% Average pore size of filter unit 1010 10 30 50 (μm) Pressure loss P1(PM0 g/L 9 12 8.5 8.6 8.3 13 m/s)

TABLE 4 Pressure loss ratio (filer at peripheral portion/ Catching andfilter at Deposited Regeneration of central state of soot 100 timesportion after the Average Central Peripheral soot soot catching for 10regeneration Portion portion 0 g/L 5 g/L hours ratio Crack Example 2-1Substrate A Substrate B 1.1 FIG. 6(b) 88% absence Example 2-2 SubstrateA Substrate C 1.4 FIG. 6(b) 90% absence Example 2-3 Substrate ASubstrate D 1.6 FIG. 6(d) 82% presence Comparative Substrate A SubstrateA 1.0 FIG. 6(a) 85% presence Example 2-1 Example 2-4 Substrate HSubstrate G 1.1 FIG. 6(b) 88% absence Example 2-5 Substrate H SubstrateA 1.2 FIG. 6(b) 92% absence Example 2-6 Substrate H Substrate F 1.4 FIG.6(b) 88% absence Example 2-7 Substrate H Substrate E 1.6 FIG. 6(d) 82%presence Comparative Substrate H Substrate H 1.0 FIG. 6(a) 86% presenceExample 2-2 Example 2-8 Substrate A Substrate I 1.3 FIG. 6(b) 90%absence Comparative Substrate A Substrate J 0.9 FIG. 6(a) 82% presenceExample 2-3 Example 2-9 Substrate K Substrate L 1.04 FIG. 6(b) 86%absence Example 2-10 Substrate A Substrate L 1.1 FIG. 6(b) 89% absence

Example 3

Even in Example 3-1, the ceramic filter 9 is basically prepared in thesame manner as in the reference example. Two kinds of SiC filters havingdifferent strengths are used in Example 3-1 as shown in Table 5. Inorder to change the strength, a higher firing temperature is set to2300° C.-3 hr, and a lower firing temperature is set to 2100° C.-2 hr.The strength is measured by a three-point bending test described in JISR1625. In this case, a lower spun is set to 135 mm, and a head speed isset to 0.5 mm/sec. This test is carried out 20 times, and the ceramicfilter 9 is prepared by selecting filters from the same production lot.In this case, the ceramic filter 9 is prepared by setting 12 SiC filtershaving an average bending load of 250 kg/cm² to the peripheral portionand 4 SiC filters having an average bending load of 350 kg/cm² to thecentral portion.

Then, the thus obtained ceramic filter 9 is wound with a heat insulatingmaterial 10 and received in a casing 8, and an exhaust gas of an engineis fed for a given time as shown in the reference example. After thecatching for 10 hours, the filter is taken out and cut to conduct visualobservation. As a result, the storing amount in the peripheral portionof the filter becomes smaller than that in the central portion as shownin FIG. 6(a).

Thereafter, a new ceramic filter 9 is again provided and the exhaust gasis actually fed. After the catching and regeneration of 100 times, theceramic filter 9 is taken out to conduct visual observation. As aresult, no crack is observed in the filter units F1. Also, the averageregeneration ratio is as high as 90%, and the soot is not existent as aresult of visual observation on the cut face.

Comparative Examples 3-1, 3-2

Even in Comparative Examples 3-1,3-2, the ceramic filter 9 is basicallyprepared in the same manner as in the reference example. In ComparativeExample 3-1, the ceramic filter 9 is prepared by using 16 SiC filtershaving a standard average bending load of 250 kg/cm² as shown in Table5. In Comparative Example 3-2, the ceramic filter 9 is prepared by using4 SiC filter having an average bending load of 250 kg/cm² in the centralportion and 12 SiC filters having a standard average bending load of 350kg/cm² in the peripheral portion.

Then, the thus obtained ceramic filter 9 is wound with a heat insulatingmaterial 10 and received in a casing 8, and an exhaust gas of an engineis fed for a given time as shown in the reference example. After thecatching for 10 hours, the filter is taken out and cut to conduct visualobservation. As a result, even in Comparative Examples 3-1, 3-2, thestoring amount in the peripheral portion becomes smaller than that inthe central portion as shown in FIG. 6(a).

Thereafter, a new ceramic filter 9 is again provided and the exhaust gasis actually fed. After the catching and regeneration of 100 times, theceramic filter 9 is taken out to conduct visual observation. As aresult, cracks are observed in the filter units F1 located at thecentral portion in Comparative Examples 3-1, 3-2. TABLE 5 ComparativeComparative Example 3-1 Example 3-1 Example 3-2 Central PeripheralCentral Peripheral Central Peripheral portion portion Portion portionPortion portion Filter unit SiC SiC SiC SiC SiC SiC Cell structure14/200 14/200 14/200 14/200 14/300 14/200 (mill/cell number) Porosity offilter 42% 42% 42% 42% 42% 42% Pore size of filter 10 μm 10 μm 10 μm 10μm 10 μm 10 μm Pressure loss 10 10 10 10 10 10 (PM0 g/L, 13 m/s) Averagebending 350 kgf/cm² 250 kgf/cm² 250 kgf/cm² 250 kgf/cm² 250 kgf/cm² 350kgf/cm² load Regeneration ratio 90% 85% 88%

Example 4

Then, the catching and regeneration are repeated by utilizing an enginehaving a slow gas flow rate in the regeneration.

Example 4-1 is an example that a ceramic filter 9 is basically preparedin the same manner as in Example 3. In Example 4-1, however, the ceramicfilter 9 is prepared by using two kinds of SiC filters having differentstrengths as shown in Table 6.

Then, the thus obtained ceramic filter 9 is wound with a heat insulatingmaterial 10 and received in a casing 8, and an exhaust gas of an engineis fed for a given time as shown in the reference example. After thecatching for 10 hours, the ceramic filter 9 is taken out and cut toconduct visual observation. As a result, the storing amount in theperipheral portion becomes smaller than that in the central portion asshown in FIG. 6(a).

Thereafter, a new ceramic filter 9 is again provided and the exhaust gasis actually fed. After the catching and regeneration of 100 times, it istaken out to conduct visual observation. In this case, the regenerationis conducted under a condition of a slow flow rate as compared with thecondition of Example 3. As a result, no crack is observed in the filterunits F1. Also, the average regeneration ratio is as low as 80%. As aresult of visual observation on the cut face, soot is existent.

Comparative Examples 4-1, 4-2

Even in Comparative Examples 4-1, 4-2, the ceramic filter 9 is basicallyprepared in the same manner as in the reference example. In ComparativeExample 4-1, the ceramic filter 9 is prepared by using 16 SiC filtershaving a standard average bending load of 250 kg/cm² as shown in Table6. In Comparative Example 4-2, the ceramic filter 9 is prepared by using4 SiC filters having an average bending load of 250 kg/cm² in thecentral portion and 12 SiC filters having a standard average bendingload of 350 kg/cm² in the peripheral portion.

Then, the thus obtained ceramic filter 9 is wound with a heat insulatingmaterial 10 and received in a casing 8, and an exhaust gas of an engineis fed for a given time as shown in the reference example. After thecatching for 10 hours, the filter is taken out and cut to conduct visualobservation. As a result, even in Comparative Examples 4-1, 4-2, thestoring amount in the peripheral portion becomes smaller than that inthe central portion as shown in FIG. 6(a).

Thereafter, a new ceramic filter 9 is again provided and the exhaust gasis actually fed. After the catching and regeneration of 100 times, theceramic filter 9 is taken out to conduct visual observation. As aresult, cracks are observed in the filter units F1 located at theperipheral portion in Comparative Examples 4-1, 4-2. TABLE 6 ComparativeComparative Example 4-1 Example 4-1 Example 4-2 Central PeripheralCentral Peripheral Central Peripheral portion portion portion portionportion portion Filter unit SiC SiC SiC SiC SiC SiC Cell structure14/300 14/200 14/200 14/200 14/200 14/200 (mill/cell number) Porosity offilter 42% 42% 42% 42% 42% 42% Pore size of filter 10 μm 10 μm 10 μm 10μm 10 μm 10 μm Pressure loss 10 10 10 10 10 10 (PM0 g/L, 13 m/s) Averagebending 250 kgf/cm² 350 kgf/cm² 250 kgf/cm² 250 kgf/cm² 350 kgf/cm² 250kgf/cm² load Regeneration 85% 85% 88% ratio

Example 5

Even in Example 5-1, the ceramic filter 9 is basically prepared in thesame manner as in the reference example. In Example 5-1, however, thereare used two kinds of SiC filter units F1 having different lengths asshown in Table 7. In this case, the ceramic filter 9 is prepared byusing 4 SiC filters having a standard length of 150 mm in the centralportion and 12 SiC filters of 130 mm in the peripheral portion.

Then, the thus obtained ceramic filter 9 is wound with a heat insulatingmaterial 10 and received in a casing 8, and an exhaust gas of an engineis fed for a given time as shown in the reference example. After thecatching for 10 hours, the filter is taken out and cut to conduct visualobservation. As a result, soot is stored at substantially the samepositions in the central portion and the peripheral portion of thefilters as shown in FIG. 7(a).

Thereafter, a new ceramic filter 9 is again provided and the exhaust gasis actually fed. After the catching and regeneration of 100 times, theceramic filter 9 is taken out to conduct visual observation. As aresult, no crack is observed in the filter units F1. Also, the averageregeneration ratio is as high as 90%. As a result of visual observationon the cut face, soot is not existent.

Comparative Examples 5-1, 5-2

Even in Comparative Examples 5-1, 5-2, the ceramic filter 9 is basicallyprepared in the same manner as in the reference example. In ComparativeExample 5-1, however, the ceramic filter 9 is prepared by using 16 SiCceramic filters having a standard length of 150 mm as shown in Table 7.In Comparative Example 5-2, the ceramic filter 9 is prepared by formingthe peripheral portion with 12 SiC filters having a standard length of150 mm and forming the central portion with 4 SiC filters having alength of 130 mm.

Then, the thus obtained ceramic filter 9 is wound with a heat insulatingmaterial 10 and received in a casing 8, and an exhaust gas of an engineis fed for a given time as shown in the reference example. After thecatching for 10 hours, the filter is taken out and cut to conduct visualobservation. As a result, in Comparative Example 5-1, the storing amountin the peripheral portion becomes smaller than that in the centralportion as shown in FIG. 6(a). In Comparative Example 5-2, the storingamount in the peripheral portion is smaller than that in the centralportion as shown in FIG. 7(b), and the difference is large as comparedwith FIG. 6(a).

Thereafter, a new ceramic filter 9 is again provided and the exhaust gasis actually fed. After the catching and regeneration of 100 times, theceramic filter 9 is taken out to conduct visual observation. As aresult, no crack of the filter unit F1 is observed in ComparativeExample 5-1. However, the average regeneration ratio is as low as 85%,and some soot retained after the burning is produced in the filter unitsF1 at the peripheral portion as a result of visual observation on thecut face. Also, in Comparative Example 5-2, crack is observed in thefilter units F1 at the peripheral portion. Further, the regenerationratio is as low as 70% on average, and the amount of soot larger thanthat of Comparative Example 5-1 after the burning is produced in thefilter units F1 at the peripheral portion as a result of visualobservation on the cut face. TABLE 7 Comparative Comparative Example 5-1Example 5-1 Example 5-2 Central Peripheral Central Peripheral CentralPeripheral portion portion portion portion portion portion Filter unitSiC SiC SiC SiC SiC SiC Cell structure 14/200 14/200 14/200 14/20014/200 14/200 (mill/cell number) Porosity of filter 42% 42% 42% 42% 42%42% Pore size of filter 10 μm 10 μm 10 μm 10 μm 10 μm 10 μm Pressureloss 10 10 10 10 10 10 (PM0 g/L, 13 m/s) Average bending 350 kgf/cm² 350kgf/cm² 350 kgf/cm² 350 kgf/cm² 350 kgf/cm² 350 kgf/cm² load totallength 150 mm 130 mm 150 mm 150 mm 130 mm 150 mm regeneration ratio 90%85% 70%

As seen from the above, according to the invention, there can beobtained the following effects.

-   {circle over (1)} In the filter of Example 1, the portion easily    retaining soot and having a fast flow rate can be rendered into a    high temperature by changing the materials of two or more kinds of    filter units to produce a difference of thermal conduction.    Therefore, the uniform regeneration of the whole of the filter can    be promoted.-   {circle over (2)} In the filter of Example 2, the flow of the    exhaust gas is made easy flow toward a filter having a low pressure    loss by combining filter units having a high pressure loss with    filter units having a low pressure loss. Therefore, the uniform    catching and regeneration of the filter can be effectively carried    out by arranging the filter having a low pressure loss in a portion    having a low exhaust gas flow amount. Also, the uniform catching and    regeneration as a whole of the filter can be carried out when the    pressure loss of the peripheral portion is 1.0-1.5 of the pressure    loss of the central portion.-   {circle over (3)} In the filter of Example 2, the uniform catching    and regeneration of the filter can be effectively carried out by    arranging the filters having a thin wall thickness in the portion    having a low exhaust gas flow amount and the filters having a thick    wall thickness in the portion having a high exhaust gas flow amount.-   {circle over (4)} In the filter of Example 2, the uniform catching    and regeneration of the filter can be effectively carried out by    arranging the filters having a high porosity in the portion having a    low exhaust gas flow amount and the filters having a low porosity in    the portion having a high exhaust gas flow amount.-   {circle over (5)} In the filter of Example 2, the uniform catching    and regeneration of the filter can be effectively carried out by    arranging the filters having a large pore size in the portion having    a low exhaust gas flow amount and the filters having a small pore    size in the portion having a high exhaust gas flow amount.-   {circle over (6)} In the filter of Example 2, the uniform catching    and regeneration of the filter can be effectively carried out by    arranging the filters having a high cell density in the portion    having a low exhaust gas flow amount and the filters having a low    cell density in the portion having a high exhaust gas flow amount.-   {circle over (7)} In the filters of Examples 3 and 4, filters    suitable for the catching amount can be set by using filters having    different strengths and arranging strong filters in the portion    easily subjected to thermal shock, so that the strength as a whole    of the filters is increased.-   {circle over (8)} In the filter of Example 5, the aggregate is    constructed by using filters having a long length in the portion    having a large fluid flow amount and filters having different length    in the portion having a small amount, whereby the catching amount of    soot can be made constant from a gas flowing section and the    regeneration can be effectively conducted.

When the pressure loss of the peripheral portion is 1.01-1.5 of thepressure loss of the central portion, the catching and regeneration canbe conducted more uniformly.

Moreover, the above embodiments of the invention can be modified asfollows.

-   a. Filter formed by arranging filters having a thick wall thickness    and a low porosity in the central portion of the filter aggregate    and filters having a thin wall thickness and a high porosity in the    peripheral portion thereof.-   b. Filter formed by arranging filters having different lengths so as    not to align both end faces of an aggregate in the formation of the    aggregate.-   c. Sectional shape of the filter aggregate is changed into a circle,    an ellipsoid or a triangle shape.

In the invention, a porous metal filter or a filter using ceramic fiberscan be used as a filter.

Furthermore, when the sectional shape of the filter is changed, only theplural filter units having a shape other than a quadratic prism at cutsection can be changed by filters having a strength stronger than theother honeycomb filter having a quadratic prism. Thus, the lacking ofthe strength in the filter of irregular shape is solved, whereby thestrength as the filter can be improved.

In the invention, the pore distribution can be made broad instead ofsharp. In this case, low pressure loss in the catching is obtained.

Example 6

This example is carried out for confirming the action and effect of theceramic filters using the above catalysts of different heat resistances.Each of the ceramic filters produced under conditions shown in Table 8(Example 6-1, Comparative Example 6-1 and Comparative Example 6-2) isattached to an exhaust gas purifying apparatus of a diesel automobile asa ceramic filter to conduct a purification test. For example, thecatalyst having a good heat resistance is used by including 10% of arare earth oxide such as CeC₂. Moreover, the distinction between thegood and poor heat resistances in the catalyst is judged by observingthe change amount of specific surface area after the heat treatment at1200° C. as shown in FIG. 9. In this test are examined the regenerationratio of the ceramic filter, temperature difference between the centralportion and the peripheral portion of the filter to be measured everyeach filter unit F1 in the ceramic filter and the durability(regeneration ratio in the regeneration of 10 times). The results arealso shown in Table 8.

As shown in Table 8, each of the example and the comparative examplesshows a high value of not less than 90% at initial stage. Theregeneration ratio after the use of 10 times is apparently high inComparative Example 6-1 having a heat resistance, but the next value isExample 6-1 instead of Comparative Example 6-3. Also, as to theregeneration temperature, Example 6-1 shows a low value of thetemperature difference as a whole as compared with the comparativeexamples. That is, the filter provided with the catalyst having a highheat resistance is arranged in the portion having a high regenerationtemperature, so that the regeneration ratio is high after the use of 10times. TABLE 8 Comparative Comparative Comparative Example 6-1 Example6-1 Example 6-2 Example 6-3 Central Peripheral Central PeripheralCentral Peripheral Central Peripheral Heat resistance Portion portionPortion portion Portion portion Portion portion Heat resistance goodstandard good standard standard standard standard good Regeneration 92%95% 90% 93% ratio Regeneration 78% 80% 50% 60% ratio after use ofseveral times Regeneration 650° C. 600° C. 650° C. 600° C. 650° C. 600°C. 650° C. 600° C. temperature

Example 7

This example is carried out for confirming the action and effect of theceramic filters prepared by using filters carried with catalysts havingdifferent active temperatures. That is, each of the ceramic filtersproduced under conditions shown in Table 9 (Example 7-1, ComparativeExample 7-1, Comparative Example 7-2, Comparative Example 7-3) isattached to an exhaust gas purifying apparatus of a diesel automobile toconduct the purification test. Moreover, the distinction of thecatalysts having different active temperatures is judged by observing arelationship between regeneration temperature and regeneration ratio asshown in FIG. 10. For example, a catalyst having a high-temperatureactivity means that when an initial filter after the provision of thecatalyst (low-temperature active filter capable of well regeneratingeven at a low active temperature) is subjected to a heat treatment at800° C. in an oxidizing atmosphere for 3 hours to lower the activity ofthe catalyst, whereby the regeneration can not be sufficiently conductedunless the active temperature is high. In this test are examined theregeneration ratio of the ceramic filter (aggregate) and the temperaturedifference between the central portion and the peripheral portion of theceramic filter to be measured every each filter unit F1. The results areshown in Table 9.

As shown in Table 9, the temperature of each ceramic filter (aggregate)is 650° C. in the central portion and 600° C. in the peripheral portion.From this result, Example 7-1 and Comparative Example 7-1 show a highvalue of not less than 90% from an initial stage. However, the ceramicfilter of Comparative Example 7-3 in which the same ceramic filters asin Example 7-1 are arranged in an opposite combination between thecentral portion and the peripheral portion shows the regeneration ratiolower than that of Example 7-1 because the activity in the peripheralportion is bad. TABLE 9 Comparative Comparative Comparative Example 7-1Example 7-1 Example m7-2 Example 7-3 Active Central Peripheral CentralPeripheral Central Peripheral Central Peripheral temperature portionportion portion portion portion portion portion portion Active low highhigh high low low high low temperature Regeneration 90% 95% 80% 83%ratio Regeneration 70% 75% 65% 70% ratio after use of several timesRegeneration 650° C. 600° C. 650° C. 600° C. 650° C. 600° C. 650° C.600° C. temperature

Example 8

This example is carried out for confirming the action and effect of theceramic filters having different carrying amounts of the catalyst. Thatis, each of the ceramic filters produced under conditions shown in Table10 (Example 8-1, Example 8-2, Comparative Example 8-1, ComparativeExample 8-2, Comparative Example 8-3) is attached to an exhaust gaspurifying apparatus to conduct the purification test. In this test, thecarrying amount of the catalyst is determined as follows. At first, theceramic filter is divided into two regions of 4 filter units of thecentral portion and 12 filter units of the peripheral portion. A ratiobetween the two regions is determined. Then, the carrying amount in thefilter is determined by equally dividing so as to satisfy the aboveratio every each filter. The regeneration ratio of the ceramic filter(aggregate), the temperature difference between the central portion andthe peripheral portion of the filter to be measured every each filterunit and the durability (regeneration ratio after the regeneration ofseveral times) are examined. The results are shown in Table 10.

As shown in Table 10, the temperature of each ceramic filter (aggregate)is 650° C. in the central portion and 600° C. in the peripheral portion.At the initial stage, the order of high regeneration ratio isComparative Example 8-1, Example 8-1, Comparative Example 8-3, Example8-2 and Comparative Example 8-2 in this order. This result issubstantially the same order as the catalyst carrying amount in theceramic filter as a whole.

When Example 8-1 is compared with Comparative Example 8-3, the sameamount of the catalyst is carried on the ceramic filter (aggregate)itself, but the regeneration ratio is higher in Example 8-1. Also, whenExample 8-2 is compared with Comparative Example 8-2, the same amount ofthe catalyst is carried on the ceramic filter (aggregate) itself, butthe regeneration ratio is higher in Example 8-2. From these results, itis effective that the catalyst carrying amount is larger in theperipheral portion than in the central portion.

Next, the results after the regeneration is repeated several times areconsidered. As a result of comparison between Example 8-1 andComparative Example 8-3 or between Example 8-2 and Comparative Example8-2, the regeneration ratio after the repetition of several times tendsto become high when the catalyst amount in the peripheral portion ishigher (Example 8-1, Example 8-2). TABLE 10 Comparative ComparativeComparative Example 8-1 Example 8-2 Example 8-1 Example 8-2 Example 8-3Central Peripheral Central Peripheral Central Peripheral CentralPeripheral Central Peripheral Carrying amount portion portion portionportion portion portion portion portion portion portion Carrying amount0.25 0.167 0 0.167 0.5 0.167 0.25 0.083 0.5 0.083 per one filter (g/L)Ratio of carrying amount 1 2 0 2 2 2 1 1 2 1 between regionsRegeneration ratio 90% 80% 95% 75% 80% Regeneration ratio after 72% 70%75% 60% 65% use of several times Regeneration 650° C. 600° C. 650° C.600° C. 650° C. 600° C. 650° C. 600° C. 650° C. 600° C. temperature

Example 9

This example is carried out for confirming the action and effect of theceramic filters (aggregates) having different carrying regions of thecatalyst carrying amount. Each of the ceramic filters produced underconditions shown in Table 11 (Example 9-1, Comparative Example 9-1,Comparative Example 9-2, Comparative Example 9-3) is attached to anexhaust gas purifying apparatus of a diesel automobile as a filteraggregate to conduct the purification test.

In this test, the catalyst carrying amount is determined as follows. Atfirst, the ceramic filter (aggregate) is divided into two regions of 4filter units of the central portion and 12 filter units of theperipheral portion. A ratio between the two regions is determined. Then,the carrying amount in the filter is determined by equally dividing soas to satisfy the above ratio every each filter. The regeneration ratioof the ceramic filter (aggregate), the temperature difference betweenthe central portion and the peripheral portion of the all filters to bemeasured every each filter unit in the aggregate and the durability(regeneration ratio after the regeneration of 10 times) are examined.The results are shown in Table 11.

As shown in Table 11, the temperature of each ceramic filter (aggregate)is 650° C. in the central portion and 600° C. in the peripheral portion.At the initial stage, the order of high regeneration ratio is Example9-1, Comparative Example 9-1, Comparative Example 9-3 and ComparativeExample 9-2 in this order. This result is substantially the same orderas the catalyst carrying amount in the ceramic filter as a whole.

The, as the regeneration ratio after the use of 10 times is measured,the regeneration ratio in Example 9-1 is larger than in ComparativeExample 9-3. Therefore, it is possible to reduce the catalyst amount inthe central portion being high temperature. TABLE 11 ComparativeComparative Comparative Example 9-1 Example 9-1 Example 9-2 Example 9-3Central Peripheral Central Peripheral Central Peripheral CentralPeripheral Carrying region portion portion portion portion portionportion portion portion Carrying region ½ 1 0 1 ½ ½ 1 ½ from end faceRegeneration 95% 90% 80% 85% ratio Regeneration 65% 58% 60% 63% ratioafter use of several times Regeneration 650° C. 600° C. 650° C. 600° C.650° C. 600° C. 650° C. 600° C. temperature

As seen from the above explanation, according to the invention, therecan be obtained the following effects.

-   (1) In the filter aggregate formed by combining a plurality of    honeycomb filter units carried with two or more catalysts having    different heat resistances, the heat resistance of the catalyst in    the high-temperature portion is improved to prevent the sintering,    whereby the aggregate can be used over a long time without waste.-   (2) In the ceramic filter aggregate formed by combining filter units    carried with two or more catalysts having different catalyst active    temperatures, it is possible to improve the regeneration ratio in    the low-temperature portion.-   (3) In the ceramic filter aggregate formed by combining filter units    carried with two or more catalysts of different carrying amounts, it    is possible to improve the regeneration ratio in the low-temperature    portion.-   (4) In the ceramic filter formed by combining two or more kinds of    filter units having different carrying regions of the catalyst, it    is possible to reduce the catalyst amount of the filter in the    high-temperature portion.

Moreover, the above embodiments of the invention may be changed asfollows. That is, heat is applied to the ceramic filter (aggregate) bythe exhaust gas in these embodiments, but the same exhaust gas purifyingapparatus can be applied to the portion of regenerating the filter bymeans of a heating means such as a heater or the like.

As shown in FIG. 13, a great amount of the catalyst may be adhered tothe wall or may be penetrated into the interior.

Next, the technical ideas grasped by the above embodiments are mentionedtogether with their effects in addition to the technical idea describedin the claims below.

-   (1) The filter is not limited to only one kind. The ceramic filter    capable of uniformly catching by changing the pore size, porosity or    the like is prepared by the aggregate of filter units as described    in the claims, whereby more uniform regeneration can be conducted.-   (2) Although the catalyst is separately adhered every filter unit in    the above embodiments, the same filter as mentioned above can be    prepared by partly adhering a seal onto an end face of one filter    and immersing into a slurry.

INDUSTRIAL APPLICABILITY

The ceramic filter according to the invention is excellent in thestrength and is excellent in the action of uniformly catching soot anduniformly conducting the regeneration, so that it can be used in theexhaust gas purifying apparatus for an automobile. Particularly, theinvention can be utilized in an exhaust gas purifying apparatus of adiesel engine.

1: A ceramic filter characterized in that different characteristics aregiven to each portion of a filter made of porous ceramic sintered bodyhaving a honeycomb structure in a flowing direction of a gas and/or in adirection perpendicular thereto. 2: A ceramic filter according to claim1, wherein it is an aggregate formed by combining a plurality of filterunits each made of a columnar porous ceramic sintered body having ahoneycomb structure and integrally bundling them through a sealingmaterial layer, and said aggregate is constituted with a combination ofdifferent kinds of the filter units. 3: A ceramic filter according toclaim 2, wherein filter units having different pressure loss propertiesare used as the different kinds of the filter units. 4: A ceramic filteraccording to claim 3, wherein at least one of units having differentcell wall thicknesses, units having different porosities, units havingdifferent pore sizes and units having different cell structures is usedas the filter units having different pressure loss properties. 5: Aceramic filter according to claim 2, wherein filter units having a largepressure loss are arranged in a portion having a fast gas flow rate andfilter units having a small pressure loss are arranged in a portionhaving a slow gas flow rate and they are integrally combined. 6: Aceramic filter according to claim 2, wherein filter units having a thickcell wall are arranged in a portion having a fast gas flow rate andfilter units having a thin cell wall are arranged in a portion having aslow gas flow rate. 7: A ceramic filter according to claim 2, whereinfilter units having a small porosity are arranged in a portion having afast gas flow rate and filter units having a large porosity are arrangedin a portion having a slow gas flow rate. 8: A ceramic filter accordingto claim 2, wherein filter units having a large pore size are arrangedin a portion having a fast gas flow rate and filter units having a smallpore size are arranged in a portion having a slow gas flow rate. 9: Aceramic filter according to claim 2, wherein filter units having a largecell density are arranged in a portion having a fast gas flow rate andfilter units having a small cell density are arranged in a portionhaving a slow gas flow rate. 10: A ceramic filter according to claim 2,wherein filter units having different strengths are used as thedifferent kinds of the filter units. 11: A ceramic filter according toclaim 10, wherein filter units having a large strength are arranged in aportion having a fast gas flow rate and filter units having a smallstrength are arranged in a portion having a slow gas flow rate. 12: Aceramic filter according to claim 10, wherein filter units having asmall strength are arranged in a portion having a fast gas flow rate andfilter units having a large strength are arranged in a portion having aslow gas flow rate. 13: A ceramic filter according to claim 2, whereinfilter units having different lengths are used as the different kinds ofthe filter units. 14: A ceramic filter according to claim 1, wherein thefilter made of a porous ceramic sintered body having a honeycombstructure is constituted in its radial section by carrying differentkinds or different carrying amounts of catalysts. 15: A ceramic filteraccording to claim 14, wherein it is an aggregate formed by combining aplurality of filter units each made of a columnar porous ceramicsintered body having a honeycomb structure and integrally bundling themthrough a sealing material layer, and said aggregate is constituted witha combination of different kinds of the filter units, and these filterunits are provided with catalyst of different kinds or differentcarrying amounts. 16: A ceramic filter according to claim 15, wherein atleast one of units having different pressure loss properties, unitshaving different strengths and units having different lengths is used asthe different kinds of the filter units. 17: A ceramic filter accordingto claim 16, wherein at least one of units having different cell wallthicknesses, units having different porosities, units having differentpore sizes and units having different cell structures is used as thefilter units having different pressure loss properties. 18: A ceramicfilter according to claim 14 or 15, wherein catalysts having differentheat resistances or different catalyst activities are used as thedifferent kinds of the catalysts. 19: A ceramic filter according toclaim 18, wherein when a high temperature gas is flown through theceramic filter, a catalyst having a good heat resistance is arranged ina portion having a fast gas flow rate or a large gas flow amount, and/ora catalyst having a bad heat resistance is arranged in a portion havinga slow gas flow rate or a small gas flow amount. 20: A ceramic filteraccording to claim 18, wherein when a high temperature gas is flownthrough the ceramic filter, a catalyst having a large activity isarranged in a portion having a fast gas flow rate or a large gas flowamount, and/or a catalyst having a small activity is arranged in aportion having a slow gas flow rate or a small gas flow amount. 21: Aceramic filter according to claim 14, wherein when a high temperaturegas is flown through the ceramic filter, a catalyst having a smallcatalyst carrying amount is arranged in a portion having a fast gas flowrate or a large gas flow amount, and a catalyst having a large catalystcarrying amount is arranged in a portion having a slow gas flow rate ora small gas flow amount. 22: A ceramic filter, wherein it is anaggregate formed by combining a plurality of filter units each made of acolumnar porous ceramic sintered body having a honeycomb structure andintegrally bundling them through a sealing material layer, and saidaggregate is constituted with a combination of two or more differentkinds of the filter units, and these filter units are integrallyconstituted by combining units having no catalyst and units carried withone or more kinds of catalysts. 23: A ceramic filter according to claim22, wherein when a high temperature gas is flown through the ceramicfilter, filter units carried with the catalyst are arranged in a portionhaving a slow gas flow rate or a small flow amount. 24: A ceramic filteraccording to claim 1, wherein the filter unit is made of a quadraticporous silicon carbide sintered body. 25: An exhaust gas purifyingapparatus characterized in that a ceramic filter comprised by givingdifferent characteristics to each portion of a filter made of porousceramic sintered body having a honeycomb structure in a flowingdirection of a gas and/or in a direction perpendicular thereto isarranged in an exhaust pipe of a diesel automobile. 26: An exhaust gaspurifying apparatus according to claim 25, wherein the ceramic filter isan aggregate formed by combining a plurality of filter units each madeof a columnar porous ceramic sintered body having a honeycomb structureand integrally bundling them through a sealing material layer, and saidaggregate is constituted with a combination of different kinds of thefilter units. 27: An exhaust gas purifying apparatus according to claim25, wherein filter units having different pressure loss properties areused as the different kinds of the filter units. 28: An exhaust gaspurifying apparatus according to claim 25, wherein at least one of unitshaving different cell wall thicknesses, units having differentporosities, units having different pore sizes and units having differentcell structures is used as the filter units having different pressureloss properties. 29: An exhaust gas purifying apparatus according toclaim 25, wherein filter units having a large pressure loss are arrangedin a portion having a fast gas flow rate and filter units having a smallpressure loss are arranged in a portion having a slow gas flow rate andthey are integrally combined. 30: An exhaust gas purifying apparatusaccording to claim 25, wherein filter units having a thick cell wall arearranged in a portion having a fast gas flow rate and filter unitshaving a thin cell wall are arranged in a portion having a slow gas flowrate. 31: An exhaust gas purifying apparatus according to claim 25,wherein filter units having a small porosity are arranged in a portionhaving a fast gas flow rate and filter units having a large porosity arearranged in a portion having a slow gas flow rate. 32: An exhaust gaspurifying apparatus according to claim 25, wherein filter units having alarge pore size are arranged in a portion having a fast gas flow rateand filter units having a small pore size are arranged in a portionhaving a slow gas flow rate. 33: An exhaust gas purifying apparatusaccording to claim 25, wherein filter units having a large cell densityare arranged in a portion having a fast gas flow rate and filter unitshaving a small cell density are arranged in a portion having a slow gasflow rate. 34: An exhaust gas purifying apparatus according to claim 25,wherein filter units having different strengths are used as thedifferent kinds of the filter units. 35: An exhaust gas purifyingapparatus according to claim 25, wherein filter units having a largestrength are arranged in a portion having a fast gas flow rate andfilter units having a small strength are arranged in a portion having aslow gas flow rate. 36: An exhaust gas purifying apparatus according toclaim 25, wherein filter units having a small strength are arranged in aportion having a fast gas flow rate and filter units having a largestrength are arranged in a portion having a slow gas flow rate. 37: Anexhaust gas purifying apparatus according to claim 25, wherein filterunits having different lengths are used as the different kinds of thefilter units. 38: An exhaust gas purifying apparatus according to claim25, wherein the filter made of a porous ceramic sintered body having ahoneycomb structure is constituted in its radial section by carryingdifferent kinds or different carrying amounts of catalysts. 39: Anexhaust gas purifying apparatus according to claim 25, wherein it is anaggregate formed by combining a plurality of filter units each made of acolumnar porous ceramic sintered body having a honeycomb structure andintegrally bundling them through a sealing material layer, and saidaggregate is constituted with a combination of different kinds of thefilter units, and these filter units are provided with catalyst ofdifferent kinds or different carrying amounts. 40: An exhaust gaspurifying apparatus according to claim 25, wherein at least one of unitshaving different pressure loss properties, units having differentstrengths and units having different lengths is used as the differentkinds of the filter units. 41: An exhaust gas purifying apparatusaccording to claim 25, wherein at least one of units having differentcell wall thicknesses, units having different porosities, units havingdifferent pore sizes and units having different cell structures is usedas the filter units having different pressure loss properties. 42: Anexhaust gas purifying apparatus according to claim 25, wherein catalystshaving different heat resistances or different catalyst activities areused as the different kinds of the catalysts. 43: An exhaust gaspurifying apparatus according to claim 25, wherein when a hightemperature gas is flown through the ceramic filter, a catalyst having agood heat resistance is arranged in a portion having a fast gas flowrate or a large gas flow amount, and/or a catalyst having a bad heatresistance is arranged in a portion having a slow gas flow rate or asmall gas flow amount. 44: An exhaust gas purifying apparatus accordingto claim 25, wherein when a high temperature gas is flown through theceramic filter, a catalyst having a large activity is arranged in aportion having a fast gas flow rate or a large gas flow amount, and/or acatalyst having a small activity is arranged in a portion having a slowgas flow rate or a small gas flow amount. 45: An exhaust gas purifyingapparatus according to claim 25, wherein when a high temperature gas isflown through the ceramic filter, a catalyst having a small catalystcarrying amount is arranged in a portion having a fast gas flow rate ora large gas flow amount, and a catalyst having a large catalyst carryingamount is arranged in a portion having a slow gas flow rate or a smallgas flow amount. 46: An exhaust gas purifying apparatus according toclaim 25, wherein it is an aggregate formed by combining a plurality offilter units each made of a columnar porous ceramic sintered body havinga honeycomb structure and integrally bundling them through a sealingmaterial layer, and said aggregate is constituted with a combination oftwo or more different kinds of the filter units, and these filter unitsare integrally constituted by combining units having no catalyst andunits carried with one or more kinds of catalysts. 47: An exhaust gaspurifying apparatus according to claim 25, wherein when a hightemperature gas is flown through the ceramic filter, filter unitscarried with the catalyst are arranged in a portion having a slow gasflow rate or a small flow amount. 48: An exhaust gas purifying apparatusaccording to claim 25, wherein the ceramic filter is made of a quadraticporous silicon carbide sintered body. 49: A ceramic filter according toclaim 15, wherein catalysts having different heat resistances ordifferent catalyst activities are used as the different kinds of thecatalysts. 50: A ceramic filter according to claim 49, wherein when ahigh temperature gas is flown through the ceramic filter, a catalysthaving a good heat resistance is arranged in a portion having a fast gasflow rate or a large gas flow amount, and/or a catalyst having a badheat resistance is arranged in a portion having a slow gas flow rate ora small gas flow amount. 51: A ceramic filter according to claim 15,wherein when a high temperature gas is flown through the ceramic filter,a catalyst having a small catalyst carrying amount is arranged in aportion having a fast gas flow rate or a large gas flow amount, and acatalyst having a large catalyst carrying amount is arranged in aportion having a slow gas flow rate or a small gas flow amount. 52: Aceramic filter according to claim 2, wherein the filter unit is made ofa quadratic porous silicon carbide sintered body. 53: A ceramic filteraccording to claim 14, wherein the filter unit is made of a quadraticporous silicon carbide sintered body. 54: A ceramic filter according toclaim 15, wherein the filter unit is made of a quadratic porous siliconcarbide sintered body.